Upgrade from MaaS 2.6.2 to MaaS 2.7
Solution 1:
I ran sudo add-apt-repository ppa:maas/2.7, making the new MAAS packages available by apt. Followed by apt update and upgrade. It just works fine.
For new installations you can use snap, but upgrading existing installations using apt should be fine as far as I know. It's always a good idea to take a backup of the database first.