I prefer a little builtin backwards (and forwards!) compatibility (by always enforcing a length for each object, to be zero-padded or truncated as needed), but yes "don't fear adding new types" is an important lesson.

That only works if you control all the clients.

Dual-publishing makes it transparent to older clients.

Obviously you need to track when the old clients have been moved over so you can eventually retire the dual-publishing.

You could also do the conversion on the receiving side without a-priori information, but that would be extremely slow.