You can easily merge a service point account with an existing standard account, even when the exact same vehicle is already registered in your standard 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 STANDARD 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 service point OBU or the one coupled to the vehicle in the standard account.

If you don't have an existing standard account, please read the FAQ "Why do I need a standard account?".