sorry you carnt do it. try to get the same deal by ring up orange and asking for retentions tell them you want the same deal or better or your off to some1 else. i do it every year with t-mobile and always get a better deal.last year it was flext 35 for £25 instead of £35 new n95 plus 5000 w/end mins to any phone.
if you want to keep the same number get your pac code move to t-mobile or o2 on a pay and go sim then move it to orange on your new contract you have up to a month to move it from the start of your new contract thats what i did when i first started with t-mobile.