We currently include a version variable already in the REST request. Look in the URI stem (before the query string portion) and you'll note there is a /beta/. The API docs note that in the future this will say "v1" or "v1_2" or something like that.
The only reason we didn't do that for the change last night was because this is beta and we didn't want to have to support multiple REST formats for the beta. But in production, we will have backwards support for earlier versions. How many versions back we'll continue to support is undecided at this point but it will be there.