Hello,
you would need to check the exact T&Cs of your company's contract with O2 as they may differ from O2's standard T&Cs for individual customers, especially since there is a generous discount on board.
As far as I know, O2's T&Cs state that you can only go down a tariff (say from £45 to £35) after 9 months from the start date of your contract, and after that you can go down a tariff per month.
It wouldn't make sense of O2 (or any business) to allow customers to take out an expensive contract and get a free (also expensive) mobile, and then let them downgrade almost immediately, as everybody would do that and O2 would lose out.
But again, your corporate tariff may be different.