funky:
If your contract finished then, they will allow you to upgrade.
As to if you will have to part with any cash - ie contribute to the new phone we will only find that out once O2 annouce what they are doing, this will only happen after Apple annouce a new device.
Although, again if your due for an upgrade now, you can just walk in and do the deal.
Personal advice - ALWAY ring, dont go into store, tell them you want to speak to retentions, your not happy blah blah blah, you been a long standing customer blah blah blah they are more flexible with what they can do if the contribution is too high.
HOWEVER, on 3G launch there was no special deal for anyone, regardless of it you were long standing customer, new customer or even a member of staff. No discount what so ever, you all pay the same "contribution" so we just have to see on that front.