Hi, my son is having trouble with hills on his bike and I'm considering replacing the 7 speed cassette on his 24" MTB from 11-28T to 11-32T (also 7 speed). I wondered if I have to replace also his 7 SIS with a new rear derailleur with more capacity. His front rings are 42-24T. Thanks for helping.

I have a 7 speed DX rear derailleur. I upgraded from a 28t to a 32t years ago. Works great. You will probably need to adjust the B-Screw. Tighten it in some to make sure the cogs clear the pulley's on the derailleur.