[cvega]

Can I change the cage of the sram XO derailleur from a medium to short cage, or I have to buy a whole derailleur?

 

[FBinNY]

They can be changed if you have or can get the parts. Everything else is the same.

 

[Stonebreaker]

I've seen the XO replacement short cage go for $100. You can get the whole X9 short cage derailleur for $69.