you might not have to....
ive just started up a 2nd contract with Orange (so i could get TP2 through phones4U) and found that i could cancel my original contract and be left with its number on a pay as you go sim
then theyll happily take the number from that and attach it to my new contract.
the only downside is I cant get the PAYG sim until my 30 days notice is up on the older contract so ive got the number from the second contract for a bit...but you could time it better, and im sure most/all operators would do it.
Currently running stock ROM on the One