Hi keasy,
Thanks for your post and for your question.
May I ask if you are currently under a Volt deal? If you choose to cancel your O2 SIM, you will keep your Volt benefits on the Virgin Media side and your package will remain unaffected.
So nothing would change on our side, however when it came to the time when your contract expires with us or you wanted to renew and you wanted to renew a Volt deal, you would need to essentially take out a new O2 SIM.
Hope that helps
Beth