A reasonable upgrade path is to do "room versioning". Each server would have a maximum supported room version, and publish it to each room they participate in, and every room has a version. When every server in the room agrees on a new version, they can publish a message to update the room version, and start talking over the new protocol. Older servers then can't join the room unless they agree to the room's new protocol.

Clients can then warn when they're in rooms with older versions than the latest supported, and since nobody wants the people they're talking to to receive scary warnings about insecurity, they'll upgrade.

And, of course, we can do similar with client versioning.