Originally Posted by
IntoTheWild
Hmmm... well if I leave it 7 speed, can I use a 9 speed rear derailleur on it?
The RD doesn't care how many speeds the rest of the system is. It goes where the shifter tells it to, as long as it has has the same cable-pull ratio, which all indexed Shimano RDs and rear shifters adhere to except old Dura Ace and the new 10-speed Dina-Sys.
If you do replace the RD, make sure it is a long-cage MTB (SGS) so it can handle the triple crank and 32T or 34T largest cog.