Olic What you can do is freeze (deactivate) the Maestro cards. As far as I know, they will be archived in your account.
You have (with the standard Premium subscription) three slots for physical cards with no additional monthly costs. Once the two Maestro cards are deactivated, two slots will open up.
Now you can order mew Mastercards to fill the open slots. A fee of (afik) €9 per card will be charged for the new cards.
My advise: keep at least one Maestro card and swop one Maestro for a Mastercard as you then have both systems in your wallet.