Enabling 2FA for an old AppleID account

I have an old Apple id and I want to migrate purchases from this old account to the new one. I have read the support page which explain the process and what I miss is only the two factor authentication for the old account. The problem is that I remember the credentials, but I don’t remember the security questions and, if I try to change them, they say they don’t have enough information for doing it. Any suggestion?