If you cancel it'll be the number of months left on your contract multiplied by your monthly line rental.
However, although you need to have had the contract for 9 months before moving it down, o2 will allow you to move it up at any time. Just give customer services a call on 0870 600 3009 and they'll advise what's available.
Like we've said, it will cost you your monthly line rental multiplied by the reminaing amount of months. What tariff do you need?
I am also confused by the OP's post.
Surely, if the OP is spending £100 a month, he would be better off paying the £75 a month tariff as it would be £25 cheaper and probably some (if not all) of the free mins and texts would cover all his needs.
But you are right, what tariff are you on OP and what tariff would suit you?