Is it possible to switch a sim from one phone to a iPhone?

I have a Motorola Social that is on a Telus Prepaid Pay n Talk plan. I want to put that phones SIM card into my sisters old iPhone that was previously on a contract from Telus but is now cancelled and has no SIM card in it. If I put my current phones SIM card in the iPhone, will I still get my current plan on the iPhone? Both are on the same phone providers and all that.

You can
but before make sure iphone is unlocked or just make it unlock
you can unlock in ebay.com

Yes there will be no problem as long as they belong to the same carrier otherwise you have to unlock it.

Yes, all you need is a micro sim card and an unlock iPhone
http://iphone.pandaapp.com/