If you want to upgrade from a 2.7 snap to 2.8, and you are using a region+rack configuration, use this command:
$ sudo snap refresh --channel=2.8 maas
After entering your password, the snap will refresh from the 2.8 channel. You will not need to re-initialise MAAS.
If you are using a multi-node maas deployment with separate regions and racks, you should first run the upgrade command above for rack nodes, then for region nodes.