Well, this would be so much easier if we knew the amount of bones sonic has compared to mario, so, (ripped right off of smashboards...) is this list! It is possible, but, most likely you will need to re-rig the model, removing 20 bones in the process. I'm pretty sure Sonic is basically just standard rigging, though, with the exception of his quills, so the only re-rigging that should theoretically be redone would be his hair(his quills), before removing the unneeded bones.
It also depends on if the bones you are editing are weighted or not. Adjusting the bone hierarchy with unweighted bones is simple and doesn't require re-rigging. Once weighed bones become involved (which is usually the case), editing the rig is also usually required.
Yeah, I talked a little bit about this earlier. Usually, though, the model doesn't need re-rigging, unless it's, say, a common biped to a sworded character, or something like that, where it's easy re-rigging, or something like Pikachu to Charizard, which is a crazy pain that would basically explode in polygons; i.e. In these cases, you have to rig the entire character from scratch, basically, unless you don't mind half the model being explody. This happened with when I was porting Feraligatr over to Squirtle, but I didn't do any re-rigging, because otherwise it would explode.
