Mobility upgrade license is recommended to be equal in number to the Mobility licenses in the deployment and can be co-termed with the Mobility licenses. Once both expire, there would be no valid licenses on the system. At this point, you do want to import the required number of Base, Plus and Apex licenses.
Depending on when your Mobility licenses end, you would consider co-terming Mobility upgrade licenses. Mobility licenses are no longer orderable, so it would make sense to purchase Mobility upgrade licenses for the same duration - whether as your first time purchase or as a renewal.
