Have you simply upgraded from one pay monthly contract to a new one?
It's not a difficult task for a customer services rep to switch you over to the new SIM if that's the case. Actually, as a former CSA, swapping SIMs was normally a wonderful call - so simple. Customer gives you the SIM serial number as printed on your new card, we enter into our computer, we all wait about an hour or so, and all is well.
But you can't have more than one SIM associated with your mobile phone number, so as long as your old SIM works, your new SIM won't work.