Switching from 8-1/4" to 8-3/4" rear ?

If the 8 3/4 is BBP all you need is to shorten the driveshaft and match the U-joint to the yoke. For those that don't know all the 8.25's are BBP.

New u bolts are a good idea unless they're fairly new already. Shock plates are the same, brake line should work fine. Springs are the same, shocks are the same, brakes are the same if the 8 3/4 is really BBP (and not just redrilled), none of that of stuff NEEDS to be changed to swap from one to the other. Although if any of it is old now would be the time to change it, when it's all apart already.