If you want to upgrade from a earlier snap version to the 2.9 snap, and you are using a region+rack configuration, use this command:
$ sudo snap refresh --channel=3.0/stable maas
After entering your password, the snap will refresh from the 3.0 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.