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.
This is a good point: Ride that bike into the ground, then fix what's broken.
But I wouldn't spend any money upgrading. It's probably wiser to save cash and upgrade to a nicer bike that already has the components you want because the cost to replace parts piece-meal will far exceed the cost of your bike.