Can this work....
Supported or not supported, give it a try.
But remember always that a successful backup does NOT mean a successful restore can be done.
Also be prepared that since it is not supported that VERITAS may refuse to assist in resolving any problems that may arise, even if it is unrelated to version differences....but some actually see it as a professional challenge and will help if they have the time.
Sometimes it simply isn't prudent to upgrade master/servers just for a rogue client.