I got a question from a customer if an older server would work with a newer client, and I wasn't able to give a good answer to this. The customer said he didn't find any documents about this. I can't either, for what it's worth.
* When can the end user expect feature degradation with...
* ... old client/new server?
* ... new client/old servers?
* When can the end user expect the clients to stop working
altogether with old servers?
* When can the end user expect the servers to stop working
altogether with old clients?
Without any guidelines, I'm going to be forced to answer "no guarantees for compatibility between versions, upgrade everything at once". That's fine by me, but we should still write THAT down so we don't surprise customers.