I have a 12 month contract, and can upgrade the handset in the 12th month, ie one month before the anniversary of the contract. So I assume you won't get an upgrade until you're in month 18.
Have a look on Moneysavingexpert.com before you upgrade. There is a good step by step plan on there for how to get more out of your phone company. The most important thing is to go through to the disconnections people, instead of the upgrade people, as they have far more power to get you extra minutes and free texts.
i've been o2 for the past 3 years and on a 12 month contract and i've never been able to upgrade early. Also if you are thinking of changing networks in the future once your 18 months are up you must first give o2 30 days notice or they'll just take it out your account even though you've been with them the whole 18 months. They get you all ways.
I agree with doglady, go through to the disconnections team they will try and persuade you to stay with them.
Im also with O2, last year i wanted the V3 because my bf then had one and i wanted exactly the same but i knew i werent able to upgrade so i spoke to disconnections team and asked for a PAC code and they asked for the reason for leaving and i gave them one.
At that time they wanted �250 for the V3 upgrade and i said i'd rather leave the network n go somewhere else where its free. They called me back and sent the phone out the next day.
Also i've ben with O2 over 5 years now and have been able to upgrade a month before its due. Im with o2 online maybe different for the o2 stores.