Originally Posted by
FunkyStickman
Correct, leave the cranks on there until they wear out, then upgrade them. I still have 7-speed cassettes on my bike, they work perfectly fine. Upgrading to 9-speed will force you to change shifters, chains, possibly cranks and derailers... almost everything.
They still make 7-speed parts, thankfully.
+1, Don't forget the rear wheel. Almost all late model 7 speed rear wheels are set up for a freewheel, not a cassette. When upgrading to 8 or 9 speed, the rear hub will need to be changed to the large freehub design. It's usually cheaper to just replace the rear wheel.