Edit, Is your old number with Sky? if it is then you won't be able to change that to your new sim and you would need to call Sky to see if they can change your current plan to your new one.
If you activate this new sim you are free to start using it whenever you want and this means changing it for the sim you already have in your phone.
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Posted by a Superuser, not a Sky employee. Find out more
If you are looking to port from another network then you can do this later - you don't have to do it at the time of Sky SIM activation. (It will allocate a number for it which you can change at a later date by using a PAC code)
I am just another Sky customer and my views are my own even if you don't like the answers
