Not only can you change out the bell housing, but it is a good idea to pull it off and change out the two front pump seals before you install it.

Pull the converter.
Shoot the 6 bolts out, should be 4 small and two larger ones, not the pump bolts but the outer bolts that hold the bell housing to the case.
Take a rubber mallet and tap the bell housing off. Be careful at this stage as you do not want to crock the out bell housing on the inner front pump housing and gouge it.
With it out of the way go ahead and replace the pump housing O rings.
Pay attention because the O rings are different, make sure you get them where they go. This is a fair amount of work if you have to do it all over again so do yourself a favor and get ONLY factory Volvo O rings. I can tell you from experience that not all rubber is created equal.
Now is the time to replace the input shaft seal as well, same advice holds here, use factory stuff.
With all of the seals replaced make sure everything is clean, clean, clean.
Now get some Silglyde or hydraulic assembly lube (Pep Boys probably) and lube up both the housing and the outer pump surface.
Getting the housing to start back on even and square can be a bit of a challenge. What you want to do is hold it up to the front pump where it is going to slide on, hold it as straight and square as possible and get a 2"x4" that will span the opening of the housing and with it straight take your mallet and hit the sucker. Inspect it all the way around as you do it to insure that you are not trying to pound it on crooked or that you have rolled an O ring.
Once started tap it gently all the way around the rim until it seats. If it seats a little out of position just tap it gently and it will rotate around until all of the bolt holes line up. Obviously you want to drive it on just as straight as you can, but sometimes the holes won't line up perfectly and then you just tap it around until they do.
Stuff the converter back in and spin it round and round until it clicks ALL the way back in. Make sure it is in all the way or bad things will surely happen when you stuff it back in the car.

Now is also the time to do any of the other seals and bushings that you might want to address on the outside of that trans while it is out and on the ground. Things like OD solenoid O rings, shift shaft seals, cooler line fitting O rings, tail shaft seal and bushings, gear selector anchor bushings, and of course do the rear main engine seal as well.

Good Luck, Mark