I've just this week got a new mobile + contract from O2 but already I've decided I want to change to another tariff. Do O2 allow this? Is it possible to do it on the website (I can't find anywhere to do it or any information about whether it can be done!). Thx in advance...

I had this same query last year.I had to call them to upgrade my tariff,as far as i know you cant downgrade a new tariff.Hope that helps.

I believe that you can change to a higher contract at any time, but you can only downgrade after 6 months, and you can only disconnect after 12 months or 18, depending on what your contract was