What to expect changing stem length/bar height

The reach will be shorter and high. More relaxed position and less bent over when seated.

it’s all personal and what terrain you’re riding.  A lot of riders how found more speed with shorter and taller setups.

I test a 30mm stem with 30mm rise and 20mm bar vs 43mm stem with 20mm rise and 30mm bar. So very close, but different and I was just more comfortable on the 30/30/20.

What you should expect from any cockpit change like this is that it's going to feel VERY NOTICEABLE at first, and then less noticeable over time. Try not to think too hard about how it feels right away, because that can be a mental rabbit hole. The key to tuning like this is to try to forget about it and get on with the task of riding the bike. If the new stem or bar stills feels odd or bad a week from now, then that's a noteworthy observation.

And if it's a different brand handlebar, be prepared to rotate the angle of the bar a couple times until the bend and sweep of the bar feel familiar. Every brand has their own unique bend and sweep dimensions, but you can really change the feel of any bar by rotating it in the stem clamp.

Shortening the stem quickens the steering inputs.  As mentioned above it will give you a more relaxed riding position.  For some people (like myself) and bikes the pros outweigh the cons.  For some people however this will make the front wheel wash more often.

I’m pretty much in the camp of go as high and  short as you can go without losing grip on the front.  Longer chainstays and a strong riding position (vertical but not too far off the back) will determine how extreme you can go.

My experience is that a 50mm stem and 20mm rise bars will have a longer steering offset (distance from center of topcap to the imaginary perpendicular plane of your hand position) than a 40 stem and 30 bars. For me, this longer steering offset gives a more oversteery feel, especially when making small corrections. Going to 35mm or shorter stem with 40mm or more rise bars feels really understeery.
 

So with the longer steering offset your front is more likely to tuck and a too short steering offset will more likely make the front push.
 

The 40 stem and 30 is what i have landed on for neutral feeling steering offset. I think the same feeling can be had with different combinations but they need a similar ratio. For example, want to run a 35mm stem, the pair it with less than 15mm rise bar. Want to run a 50mm riser bar, then probably best to pair with 45 or 50 stem.
 

These are generalizations that are really a function of bar sweep and roll relative to stem length. This is my experience and other riders will likely feel differently.

I would love to hear more on the bar angle thing that the B-Practice podcast dove into the other week. Their comments on front wheel weighting/pressure got me interested. I've been having a hard time going from my 2017 pivot 5.5 to 2022 stumpy evo mullet and getting the front end of the cockpit right. 

What were your old bars? The Renthal bars have less backsweep than most, so there may not actually be that much change in the fore/aft location of your hands, if your previous bars had a more typical backsweep.

I recently swapped from a 50mm stem to a 45. A shorter stem should theoretically reduce the amount of steering “flop” or oversteer in corners. The head angle on my bike is fairly slack at 63.1 degrees, so I was getting a fair bit of oversteer/flop in bermed corners. Swapping the stem definitely reduced the steering flop and in some cases made it feel easier to get the bike on its side knobs. It also feels fantastic on climbs. However, the shorter stem reduced the amount of weight on the front wheel so it feels like it wants to wash a bit more. To correct this I find I have to ride in a bit more of an aggressive hinged position. Not sure if I prefer the 45 over the 50, but it’s definitely worth experimenting and learning the pros and cons of different set ups. 

I like my bike high and short, due to my riding style, my body, and my back. I have long legs, and a short torso, and like having the bike pretty much as nimble as possible. BMX and freeride background. 

I run either 40mm or 50mm rise bars on all my bike, including my downcountry bike.

32-38mm stem, with typically at least 20mm spacers underneath. 

Renthal or Tag Metals bars, always, because of their 5* upsweep and 7* backsweep

I am fit for a LG or even and XL, but would prefer a long MED or a short LG. I would almost always ride a MED, but my legs are too long for some and hit the bars. A reason for the shorter stem on the LG. 

I have always run higher than normal setup, but recently it got much higher after a back injury, and the need to keep a much more upright riding position when climbing. 

The biggest drawback for most people is a wandering front wheel when climbing and not getting enough traction when cornering. However, riding a short bike, I can have a more dynamic riding position and move around the bike better. 

Shorter higher setup helps with your ergonomic position on the bike more than anything. Definitely helps with jumps and jibs too, which is most important. 

It’s not that trigonometry is beyond the abilities of mountain bikers (other than maybe the youngest among us), but for whatever reason these conversations are mired in numbers that don’t really tell you what they look like they tell you. Whether you care about where your grips end up in space relative to the bottom bracket or the steer tube or the front axle or the saddle, none of numbers most easy to grab and post online (e.g. stem lengths, bar rises) get you there. And the numbers themselves (“45 mm”) can be freighted with all sorts of riders’ personal baggage from association, as if “45 mm” has some special property it carries from bike to bike, rider to rider (other than this hunk of material measures this long center to center).

I would consider finding a convenient wall you can mark up, and maybe something on the ground to chock a wheel against for repeatability, and mark on the wall where the center of the BB is, the center of the front axle, and the center of the grip against the wall. Then swap stem or bars, make a new mark, and see how much it moved. Tape a piece of graph paper to the wall for convenience? Ride the new setup, take notes. Repeat?

I am informed by my motocross background more than my MTB experience. I've noted in another thread that I always felt like the bicycle was still very oriented toward the XC credo: low bars, tall seat, wear spandex and pedal hard! Even my slack enduro bike has a much lower handlebar position than my MX bike, compared to where my feet reside.

On the MX bike, I always went with a flatter handlebar bend, positioned neutrally on a forward-mounted bar clamp. I have long arms and shorter legs, so getting the bars out of my lap was a priority. (MX bikes don't get sized by frame, so we riders have to adjust whatever we can.) 
On the MTB, I haven't made too many changes, but I'm looking at taller bars. For my next bike, I may size up to an XL to get more stack height. I'm comfortable with a longer reach. 

Also don't discount the impact of even 1 to 2 degrees of sweep between your old and new set up and its influence on reach numbers. I recently switched to a set of 12 degree backsweep handlebars after riding 8 and 9 degree backsweep bars for nearly my entire time on bikes outside of BMX. I went with a 38mm length stem with the new bars which also had an additional few millimeters of rise in comparison to the old handlebars I was running on a 40mm length stem. The combo of additional backsweep and rise plus -2mm of stem length shortened the reach on the bike which had previously felt too long enough that it threw off some of the benefits for wrist positioning I was hoping to gain in opting for more backsweep. I've gotten used to it over time but now am considering a longer stem and shortening the bars -10mm to achieve my desired wrist position.  

went from a 50 mm stem to a 37 mm and definitely prefer the 37 because of the more direct steering feel. Also experienced some loss in front wheel traction. Since I have flip chips in my chainstays, I went from short to long setting (+5mm) and this definitely helped boost my confidence on loose open corners and I didn't need to adapt my position on bike too much. My head angle moved from 64.5 to 64 while doing so. Regarding wheel flop: really cannot tell a difference in wheel flop between both settings. 

Experienced riders end up with a cabinet of stems and bars.  That's life, man.

You try one thing and feel how it works on climbs/descents for you, and then you adjust.  If your changes lightens the front end too much, either drop the rise/spacers or extend the stem.  I might ride a frame that's too long for my height, but with long-ass arms, it works the way I have it set up.  I have 30mm of spacers and high rise stem(DMR Defy 50) and 50mm rise bars on my L Ripmo.  Sure I have to shift my weight forward if it's really steep, but that's fine by me.  I'd rather my weight bias be a little back than a little forward.  I climb so I can come back down.

Feeling the difference in weight bias is the most important because once you know the range of how things feel, you can dial in really well.

The taller the position on the bars, the more leverage you have on picking the front end up,

as I find bikes getting longer with more wheelbase, the bars also need to come up in height for the bike to behave how I like it too.

What I've noticed switching from the longer/lower setup to a shorter/taller one is mostly what everyone else has said, primarily more upright/relaxed, which usually benefits most bikes I ride as I'm 6'4" with long limbs and a relatively short torso. But I've also noticed an increase in pressure on the rear wheel as a result of the more upright/rearward position and, in some instances, a more distant feeling on the front tire.

I experimented this week with going 10mm taller on a bike I felt had plenty of stack height to begin with, expecting it to improve what I liked about the bike instead, it reached a point where I had difficulty trusting the front wheel, and I felt like I had to back the bike into turns with so much of my weight over the rear. 

Like @Explodo said, expect to end up with a few different options if it doesn't end up being what you're looking for. I usually change one thing at a time until I find the sweet spot and then keep the setup where it is. 

Good point on the weight bias moving around as you change the stem/fork.  If you're a particular person, expect to be adjusting shock/fork pressures by a few psi if you change your weight bias much.

and that's the point where the longer chainstays helped me. So now I can have a more relaxed cockpit setup and still got the pressure I'd like to have on the front 

Totally agree, the bike I recently went taller on is about 5mm shorter rear center than the bike I normally ride with the same bar height.