Can i port a new number to ID and retain the old number which is currently active?

I have purchased a memorable number which is retained on a PAYG SIM from a different mobile service provider. I would like to use a PAC to transfer this new number to my ID Mobile pay monthly SIM, but I would like to retain access to the number currently used on that account. The reason for this is that the current number is one I have used for a long time and it's on record with a multitude of online retailers, financial institutions and local services etc. as well as friends and relatives. I will do my best to update everyone with my new number, but it's inevitable that I will miss some and so I would like to keep the old number on a PAYG SIM for a while until such time that I'm confident that I have managed the transition completely. Is this possible, or is the old number always lost if a new number is ported in?