It really doesn't matter. However if you are unable to change the phone number yourself via my O2 Settings > Phone> my number, save and reboot the phone.... then give O2 a call and they will reset your account. All numbers in this guide Guide: Coronavirus Community Help and Support
Veritas Numquam Perit
It's unlikely that customer service will be able to change it.
Make a note of your IMEI which is on the phone box anyway, in case you need it.
You may find that your MyO2 will update itself eventually anyway.
>I think you're right, that MyO2 might update itself eventually.
I wouldn't hold my breath.
MyO2 is still showing my phone as an HTC one I stopped using about 4 years ago. No way to change the details. I've tried deleting the 'my Device' and then re-adding my phone no. But it still comes up as the old HTC.