Originally Posted by palincss

Any Shimano MTB 7, 8 or 9 speed rear derailleur would work here, as the cable pull is all the same. Shifters are a different story: for indexing with an 8 speed cassette you need 8 speed shifters, and for indexing with a 9 speed cassette you need 9 speed shifters.

However, there's a question about the hub and specifically the freehub mechanism. You can't put an 8 speed cassette on a 7 speed freehub, it's too wide.