FBB_Gabriel,
Thank you for the additional information. To start, you would need to have access to the account. This means knowing or reseting the password. You would do that by following Change your Apple ID password. If you have access to the iPhone that is connected to the Apple ID, try to use it when following the steps to recover your password.
It is the case that you cannot merge accounts or use the same email for both accounts. If you follow Change your Apple ID, you can change the email to an iCloud email address. Just be sure to update your information for your a rescue email address About your Apple ID email addresses or if using two-factor authentication, that your rusted phone number is up to date Two-factor authentication for Apple ID.
Once you have access to the account, to change your nickname, you would have to do this in iTunes Set up and view an iTunes Store account on PC or the Music app Set up and view an iTunes Store account in Music on Mac.
- Go to Account > Sign in and sign into the account (sign out of the current account if necessary).
- Go to Account > View My Account...
- Scroll down to the bottom to the Settings section and change Nickname here.
As you cannot merge accounts, if you want to share purchases from this account to your new account, you would need to look into Family Sharing. Check out overall information about What is Family Sharing?, and the YouTube video from Apple Support about it How Family Sharing works. Also see:
Cheers!