http://astrillrcpusdryujyzbsl7wsugvyfc5s6veyrpkojiqbrorgkfdxvid.onion/features/vpn-protocols/wireguard
(This is in contrast to a crypto agile negotiation, in which client and server must negotiate and agree on every single primitive and key length, piece by piece.) Once an admin is certain all of their clients have been updated to support the new protocol, they could simply disable the 1.0 protocol at the server and be done with it.