You don't need a pac or cancel if you want to change to a SIM only tariff.
Just go to the upgrade option in your MyO2 and choose a SIM only upgrade.
No notice period required either.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
Currently using: Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)