I got it to work, somehow, here is what I have done prior to setting the feature compatibility to 5.0:
just for safety, a mongodump:
snap run rocketchat-server.backupdb
stopping the snap:
snap stop rocketchat-server
refreshing diretly to a new channel:
snap refresh rocketchat-server --channel=7.x/stable
console output: rocketchat-server (7.x/stable) 7.0.0 von Rocket.Chat (rocketchat✓) aufgefrischt
restarting rocketchat-server
snap start rocketchat-server
Seems to be running, for some strange reason, even though the service is disabled, masked in systemd and disabled in snap, caddy is still running, even though I have stopped it prior:
snap info rocketchat-server
rocketchat-server.rocketchat-caddy: simple, disabled, active