How to switch iPhone SIM cards?

So I have an iPhone 3GS but I'm about to purchase a used iPhone 4 by owner with the same provider I have. I want to give my 3GS to my sister also, who has a simple phone with easy access to her SIM card.
How would I switch the SIM cards? And since my sister doesn't have a data option, can she quickly add a data plan with AT&T when she receives my iPhone?

This might help? Sorry if it doesn't.

Yes she can get a data package added to her line. Now as of your Sim card the 3gs takes a normal Sim card and the iphone4 takes a micro Sim card so you will have to goto att so they can give you a new sim card.

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