Peirre, I’m sure that your old tag will still be valid if there’s credit on it, you’d be best getting in touch with them.
online at gov.uk/dart-charge
over the phone by calling: 0300 300 0120
Found this too
"Existing DART-Tag accounts
We sent you personalised instructions using your DART-Tag contact details before we introduced the changes, telling you how to transfer to Dart Charge
If you have not transferred, your account might not work under the new arrangements. Please call the Dart Charge team on 0300 300 0120."
I had one for my car when I lived in the UK and I’m sure I got one of these letters to say I had to create an account, was a long time ago.