Sign up a user that already exists

I'm trying to create a flow where a "guest" has filled out am appointment form but hasn't created an account. I am saving their details in the user table (email and name).

What's the best way of transforming the "guest" user to a fully fledged account when they sign up?

It would be nice if I can sign the user up, but keep the same user entity since I'm using it to link to other tables to keep track of what they've booked.

When using the better auth signUp I get the User Already Exists error:

[APIError: User already exists] {
status: 'UNPROCESSABLE_ENTITY',
body: { code: 'USER_ALREADY_EXISTS', message: 'User already exists' },
headers: {},
statusCode: 422
}
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