Yes, you can! But read the full KB article before taking action.
Simply cancel the MyGet subscription in the Azure Marketplace: this is not a delete, and will downgrade your MyGet subscription plan to the MyGet Free plan.
Any private feeds will remain private.
Next, login to the MyGet web site using your existing MyGet account, and upgrade your existing MyGet subscription to any of the paid plans on https://www.myget.org/plans.
Unfortunately, changing your MyGet username is currently not supported.
If you feel strong about it, you can create a new MyGet account, upgrade that one instead, and transfer feed ownership to the new account.