I'm with O2 and have had no problems upgrading. They only check your payment history with them, and if it's ok then you can upgrade.
I don't think any company will take you on for a new contract while you are in an IVA.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk