If you have created a guest client at some stage and then your client wishes to become an app user, you can merge the accounts together. Once you do this it will remove the guest client and you will be left with only the app user client. Any reports or invoices you did for the guest client will be transferred over the app user profile.
Your clients can also help to avoid this step by following the instructions in this video where we show them how to become an app user when their email address is already in use on a guest account.