Hi Leongreen84,
Thank you for your post and welcome to the forums.
At any point during your contract period with your handset - you can use any SIM you want in there. The SIM part of the agreement is on a 30 day rolling contract - it's possible to change the package you are on. Alternatively if we couldn't offer you one that suited your needs you would be able to take your number to a different supplier with your PAC code - that would cancel down the SIM this side, but the handset agreement would still be active 🙂
Cheers,
Ryan.