iPhone 4s - replacement SIM & PAYG

snow_ball

I recently lost my contract iPhone 4s (uninsured) and have now bought a replacement through my existing network provider but as well as a replacement SIM, I have had to buy a PAYG SIM.

Apologies if this is a silly question, but how do I make use of the PAYG SIM which will obviously be on a different number? Do I just insert the SIM, activate it and use up the mins/text/data after backing up from iTunes and then go back to my original Pay Monthly SIM/number?

Thanks for reading!