on 31-01-2021 15:31
on 31-01-2021 15:37
If you did an upgrade you can carry on using your old sim.
If you took a new contract, you will need to cancel your old contract and migrate your number Guide: Migration & porting into O2
Your old sim won't be 5g, so if you need and can access 5g, you'll need to use the new sim anyway.
on 31-01-2021 15:38
You can either carry on using your old sim
Or you can swap the sim to the 5G sim