I reccomend you keep the 27" wheels, maybe buy aluminum ones, though. There may be less tire selection, but how much are you really going to spend on tires anyway? Selection is not important unless you're going to be doing more than riding around, and to be honest, I don't think old Schwinns are really suited for racing.

27" wheels are superb for commuting and fun riding, fast riding, etc. They may no longer be the de facto standard of racing, but they are extremely comfortable and are much more pleasant to ride on both downhill and uphill, IMO.

And no, don't switch the bottom bracket. The sealed ones are a newfangled "throw away part" craze, "buy and replace." Well, in [x] years, when Shimano isn't using that style or sizing or whatever of BB cartridge, you'll just be at square one, starting all over again buying a new sealed BB unit.

It's quite a sinister effing marketing scheme.

Keep the old BB. If it starts acting up, open it, re-grease it, and keep riding, indefinitely. You'll never be obsoleted, until bike shops stop using grease and forget how to take apart BB's. ("Introducting Shimano's new self-installing and removing BB! Remember those tired old threads? Don't even do the work yourself!")

Anyway... Switch for Aluminum 27" rims and keep (but regrease, etc) your current BB. That's just fine.