Transfer from Anonymous to Real User

I am trying to transfer from an Anonymous User to a Real User. The only methods that I use inside my Flutter App to log in are passwordless(magicLink and oAuth). After trying multiple workarounds to convert the user, since there is no direct method to do the conversion, I've decided to create a normal account with an anonymous flag which I can update once the MagicLinkLogin is happening. Basically I have a user with a userId that is logged anonymously and then when I send the magicLink with that userId and the new email I want to update with the new email. However I am faced with an issue in which I can't send the email because I get an error: "user already registered, use another id or a unique one" or something like this. The docs says: If the provided user ID has not been registered. I tought that if I provide the existing ID it would know that I just want to login somehow. Does anyone has a way of converting from anonymous to real user or is there a simpler method to do so which I've missed?