You can change the subscription type for any of the created users at any time from the Microsoft 365 Control Panel. In the "Microsoft 365 Accounts" module, choose the user you want to modify and click on the "Modify account" icon. From here you can change the first and last name of the contact associated with the user/email as well as the account type for that user (Exchange Online, Microsoft 365 Business Basic, Microsoft 365 Business Standard). You must have available and unused licences for the account type you want to switch to in order to make the change.
During the switch, the user will not lose any mail, but depending on the type of account the user changes to, the available functionalities will vary.