You need your PAC from Tesco.
Guide: How to get your PAC [June 2021 Update]
Scroll down to where it says 'To port into O2'.
You'll need to contact O2 customer service.
Call 0800 032 1402 from any phone.
When it comes to the reason you are calling, stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
Say 'Upgrade', 'Lost/Stolen', or 'Fraud' if the suggestion above doesn't get you through.
The 0800 number isn't available until 9 am.
Explained in the links above @iga-52
Track usage, upgrade and get support with the My O2 appDownload My O2