[chrismorgan]

Ah, I see the issue you’re going for now: feature evolution. Certainly that’s the trickiest part of the scheme. I still wouldn’t say end-to-end encryption is special here; certainly it’s likely to be a more intrusive update than most, but if introducing it late you’d still handle it the same way as any other, most likely as an optional feature for a certain period of time, after which incompatible clients can no longer connect.