new crank, old chain
i just installed a new crank on my bike. I know that it's a good aidea to change the chain anytime you change the cassette.
How important is this when changing the crank?
Totally depends. I would start by checking your chain with a tool and seeing how worn it is, and inspecting your cassette as well. If either one is fried, there you go. Otherwise, have a look at how the chain sits on the chainring. Especially on a big 52 or whatever, you can really see if it's worn and the pins don't want to sit in the center of the notches between the teeth.
Listen to me
Chains are cheap.
Why not just replace it?
Chains are cheap, but a new chain and really old cassette often run worse than stuff that has worn together, and he may not want to replace both right now.
Chainrings, unless very badly worn and the teeth are obviously hooked, are pretty tolerant of chain condition. I've put brand new chains on chainring sets with 25,000+ miles on them and they run and shift just fine. Casettes are another matter altogether.
i would just change the chain just to be on the safe side pal.