Jedi is correct on the way it needs to be handled. Social login for all level0 accounts. Then they can upgrade but they first need to get a free account.
AccessPress Social Login since it uses the users email will connect an already established paid account under one rule. The rule is paid member must be using the same email address for both your site and social account.
I have tested this and it does work with AccessPress Social Login. If the email is not the same though they will end up with two accounts and then email about the account not being correct.
I have found most users stick with their original setup. If they use social they always use it, if they started with email or username they remain user or email based login users. I have only had one user that created a second account based on the social account not having the same email. I was able to bind the two accounts but I would make a FAQ about the social login so people can answer their own questions on the subject. Although I have not made a FAQ about it but if I got more than 3 or 4 I think I would make it so I could refer them to it.
Ross