So the migration looks like "upgrade the client or you won't be able to connect to the server any more"? What if you use the client to talk to multiple servers, some that use the old version, some that use the new version? Maybe via a config variable adjustable per server? Then you do out of band version negotiation, and you might get away with this in the VPN setting, where entering arcane config vars is commonplace, but not in e.g. the TLS setting.
I guess that's thanks to the fact that WireGuard is a new system and new systems have little legacy bloat. Maybe the WireGuard author had golden hands, and the system is perfect, and indeed it it is quite good, but I think instead that WireGuard will eventually require a new version. Then one such solution will have to be chosen.
