The process of retaining your line and replacing it with a new connection is called a line replacement.
Unfortunately due to a recent change in policy, this is now not a possibility if you are billed by the network directly. The only way you are able to retain the number in question is to pay the termination fees which could be extremely expensive.
If you are billed by Ice comms directly rather than the network then this is still possible providing you are on the o2 and Vodafone network. There is a fee of £45 per line to complete this however.