Q:
Serial code overlap
You can not link two Guild Wars 2 accounts. You purchased a new Digital Deluxe Edition account.
The only way to upgrade an existing account is with the upgrade option found in the Gem Store for 2000 Gems.
You can make a new account with that Serial Code. If you experience any issues using that Serial Code after trying to apply it to another account, you can contact the CS Team for assistance. Top/Bottom of Page – Support – Submit a Request.
Good luck.