Perhaps a (signed) version field would be useful, to allow for future breaking changes.
+1 for the version field in the signed data.
Not sure how it could be made so a gradual changeover would be possible, a version field would only allow gradual upgrade of clients, once a server is upgraded, everyone gets v2 messenges...
Also do you have an idea about how it should be implemented in existing systems, where users are already using the old system ?
We have to support both variants, our own and the new standard. I am thinking to add a setting where user can check "old proprietary (obsolete) callback" and "callback according to standard". With the latter being the default for all new clients.
So existing configurations will continue to work while all new users will per default get the new standard callback.