Hi!
Yes, if the client doesn't support redirects, it'll work as before - that is, the client will be kicked out when the server is shut down and then the client will connect to a new server because, of course, if there's failover, there must be some way of telling clients what server to connect to. The error will be ER_SERVER_SHUTDOWN which is what old clients were always getting in this case. So, basically, for old clients there will be no changes whatsoever.
..which is not ideal. Some backwards compatibility would make the user experience much better.
Hence, I still feel that Daniel's submission [3,4] would be a better design, being more robust and safer. An error+redirect is easier to reason about regarding integrity of data and error modes than the voluntary maybe-redirect that is now in the works by Yuchen.
Hm. Technically, the server can check whether redirect_url is set, and send ER_SERVER_REDIRECT instead of ER_SERVER_SHUTDOWN. I'm not sure it'll be much of an improvement, but it's doable.
That would make sense from the onset to ensure the rollout of the redirect feature is successful and users (and connector authors) can discover what happened.