You can easily merge a fast-track account with an existing full account, even when the exact same vehicle is already registered in your full account.
You can do this by following the “I’m driving” process:
1) Log in via your licence plate number, the country of origin of the vehicle and the PIN code you received when you got the OBU. Choose: "ADD VEHICLE TO MY FULL ACCOUNT".
2) Afterwards, log onto the Road User Portal for a second time, this time with your username and password. You can then decide which OBU you want to keep coupled to the vehicle: the fast-track OBU or the one coupled to the vehicle in the full account.
If you don't have an existing full account, please read the FAQ "Why do I need a full account?".