Ok had a question for those iphone users out there. In short can I purchase/activate an iphone and then put in another SIM card and have all functionality?

Reason I ask is that my wife wants an iphone. Well we are on a family plan and her line can not be upgraded for another year were mine can be upgraded now. Well techincally hers can be upgraded now but it's an extra $200. I was planning on just activating the iphone on my account, taking the iphone home and putting in her SIM card. I know it's a little more difficult to changing out the SIM cards on the iphone but I think I can do it. However the guy in the store said that the iphone is tied to the SIM card that activates it and I would not be able to use another SIM card in it? Is this true? Obviously I am trying to circumventing the extra $200 charge which is what made me leery. If so then this means that you can not sell or buy a used iphone with out paying for your carrier to activate it to your SIM card?