The same way you'd handle it on a per-header basis . . . you just use the new API version in the URL where needed. Sorry, I feel like I must be missing something here.
I think for a lot of people the reason they don't see the benefit of things from REST is they try to use them in isolation. You say "this is useless for my RPC style API!" and you are right.
FWIW, I've made an effort to follow the purity of it, but the APIs I see that try to do true ReST are insanely obtuse. If you know of any real world case studies where a provider went from "fake" ReST to true ReST, I'd love to read it. The contrived examples are not helping me out in the comprehension department.