Customer service needs to remove your old watch first.
This must be done on your account by them.
All contact numbers we know of here Guide: Coronavirus Community Help and Support
or advice from an O2 Guru to get through:
If you call 202, 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 you are calling about an upgrade or fraud to be connected and not cut off.