Used Bike Upgrade Priorities

Upgradeitis can be a very costly affliction. From a strictly financial perspective you're better off dropping a few bills in a jar everytime the urge strikes you, then eventually you use that jar as a starting point for buying another bike instead.

What you can do which isn't entirely stupid money-wise, is to switch to nicer parts when an item needs replacing for other reasons. But yeah, many riders keep a 2nd wheelset at hand. Don't expect any overwhelming improvements though.

i have been moving Shimano 600 brakes for the last 15 years over different levers and frames and they still work great !
also Shimano 600 hubs are right now on my spare wheels working great too,
i did upgrade the old Shimano 600 crank and Bottom Bracket into Ultegra say in year 2004 more or less, when the new design and standard came in, then finally moved into 9 speed when a friend of mine bought my 8 speed levers, derrailleur and cassette,

spare wheels are handy, they will keep you at all times on the road (when you need to service or maintain a wheel), give you for example the flexibility to choose and swap from training tires into competitive tires forth and back, like the guys in F1 do, or install two different cassettes, one for climbing and one for flats,