Here's my personal opinion.
Everyone should be entitled to a review of something that happened seven years ago. Particularly, as it was not considered serious enough to bar access to the network.
Someone at giffgaff may have put a note on your account that was inaccurate. The fact is, as members of the public, we just do not know.
Your first action should be to ask for an email review via email@example.com
You should receive a reply within 5 days.
If you are not satisfied with the email reply, then under GDPR you can request free copies of data held by giffgaff about you. Upon receipt you have a right to request the correcting of perceived inaccuracies. There are further remedies available should the outcome not be to your liking.
Lastly you say you have not been able to update your seven year old sim.
Pop down to your local retailer checkout (e.g.Tesco, Poundland, Sainsbury, Asda) buy a 4G, 3in1 giffgaff sim. You can do a sim swap in around 30 minutes and your mobile number, account details, credit and goodybags will all be put on your new sim.