Replace BB when installing new cranks (Shimano)?

You can use the existing one...it will eventually wear out and you'll have a back up.

Yup you can...

the external bb for the LX and XT are the same part number. The XT is going to come with new bearings and cups so just hang on to them for spares. As mtbiker72 said, you'll need em eventually.

Good Dirt

Don't fix what ain't broke.

Assuming the old BB is working and compatible with the new crankset, no benefit could be derived from replacing it, so leave it alone until it needs replacement. Many people remove perfectly good components, and carefully put them in the basement, figuring they'll use them later on either as a backup, or on a "B-bike" The reality is that most of this time these are never used again.

Save your dough and leave the old one in place, and save the new one for when you need it.

Cool, thanks for the help. That's what I was hoping. I ride through a creek on a somewhat regular basis so I'm sure that will shorten the life of my BB.