Synapse 1.0 will default to new room version (v4), this means that any new rooms that it creates will be of version v4.

Generally this is a good thing, but there will be cases where a given server wants to create rooms with old room versions. Perhaps they need to interoperate with another server that for whatever reason cannot upgrade.

So there needs to be a way to override the room default version and replace it with an older version - even an unstable version.