Your handler would have to intercept the call before Axis attempts to dispatch it. Otherwise you'll get the classic "no such operation" error for one or the other version. I also don't think Axis will let you deploy two services at the same URL. You would do better to deploy an intermediary (see the Synapse project) at the original URL, and then this intermediary can route the message accordingly. Your intermediary would need to use the message style service. It's definitely a good practice to make upgrades as seamless as possible.
This was first published in November 2005