Swtch between 11-32 and capreo?

Why?

To specifically answer the questions you asked: The chain will probably be ok as long as you carefully avoid cross-chaining. You'll know as soon as you have the new wheel on, and you probably have 2 or 3 links you can safely remove anyway. The derailleur will need a little tweaking, but probably not much - the hubs won't have exactly the same spacing, so you'll have to tweak your barrel adjusters and maybe your limit screws.

I'm not going to try to talk you out of it if you're just doing it for the sake of doing something different, but there are better ways to accomplish what you're trying to do. A 9 tooth cog is a huge gear on a 26 inch wheel. What chainrings are you running? I used to run a 46 tooth big ring and could do about 33mph in the 46-12, my biggest gear at the time (on 700c wheels). A 44-11 would be actually a slightly higher gear, so the same rpm gets me to 34+ mph. Even if I adjust that for skinny tires on 26 inch wheels, we're still looking at 28-30 mph. How fast do you need to go?

If I can chime back in here

I'll expand on Shiggy's statement there. Assuming you run a 90rpm cadence and are using 1.5" smooth tires on 26 inch mtb rims, a 44-9 combo would equal 32.4 mph. 110 rpm would put you over 40 mph. You probably couldn't generate enough torque to spin either of those rpms unless you were headed down a steep hill. . It's just too big a gear.

Not to mention the extreme stress you're going to put on your drivetrain. You're going to be putting all of that torque through 4 teeth on the cogset. That's it. That's a lot for 4 little teeth to handle. The smaller the cog, the smaller the chainring, the greater the wear.

Do you have any road bike experience? The comment you made about the 9-26 having close spacing like a road cassette doesn't jive - I ride a 12-23 on my road bike, and the widest range I know is a 12-27, which is still significantly smaller than the 9-26 of the Capreo. A 26 inch wheel with a 1.5" tire is only 5% smaller in circumference than a 700c wheel with a 23mm tire, and you're going way overboard trying to compensate for the difference.

Here's the best concrete example I can give you. Time Trial bikes use wheels that are basically the same diameter as mountain bikes (ok, they're 12mm bigger). A rider in the Tour de France uses an 11-21 cassette and a 53 or 54 tooth chainring, so their top gear is about 4.9:1. Your 44 tooth chainring and 9 tooth cog are about the same ratio. So if you can push the same gear that Jan Ullrich does, go for it. Otherwise it's just too big a gear.

I hope you know we're not trying to beat you up, this just isn't a feasible idea.