Iphone sim swap question

[Khurram]

Hey guys...
so here's the situation.
My wife and I are currently using 3g's and are on the 3g att plan. My younger brother is still using the 2g and is on a 2g price plan.

One of his friends wants to sell him his 8gb iphone 3g because he wants to upgrade to 16gb.

If my brother buys it from him...can he just swap out the sim from his old iphone 2g and use it in the 3g without having to upgrade/renew/pay the 3g price plan from att?

Since we are all on a family plan he wants to still be on the 2g price plan but use the iphone3g. If he puts in his old sim will he be able to use the 3g since it's already been unlocked by att or will they start charging him at the 3g price plan when he puts in his sim?

Thanks in advance for the help.

 

[Rayfire]

Here is a part of the Terms & Conditions from AT&T

Data Plans: An eligible data plan for iPhone is required. This data plan covers data usage in the U.S. and does not cover international data usage and charges. If it is determined that you are using iPhone without an eligible data plan, AT&T reserves the right to add an eligible data plan to your account and bill you the appropriate monthly fee. © 2008 AT&T Intellectual Property. All rights reserved. AT&T and the AT&T logo are trademarks of AT&T Intellectual Property. Subsidiaries and affiliates of AT&T Inc. provide products and services under the AT&T brand. © 2008 Apple Inc. All rights reserved. Apple, the Apple logo, iPod, iTunes, and Mac are trademarks of Apple Inc., registered in the U.S. and other countries. iPhone, Multi-Touch and Safari are trademarks of Apple Inc.

You might get-away with a few billing cycles for your younger brother's phone YMMV, but like what the terms and conditions say, if they catch you without an appropriate data plan, they can add it.