Sprint doesn't charge to change plans, however, they might charge an activation fee for the new phone which might run $20 or $30... Call and ask, if you have been a customer for a few years, they will probably waive the fee if there is one.
Keep in mind, if you have a simply everything plan as a result of a new phone purchase, and it was a "smart phone" that required unlimited data, like the Hero/Moment, Blackberry etc., they are NOT going to let you drop the simply everything plan if you are still under contract for the phone, even if you change to a different phone... They have the money invested in subsidizing the expensive phone to you, and are expecting to recoup the cost of the subsidy with that Simply Everything plan they required you to purchase along with the phone. I'm sure you can "buy your way out" if that is the case, but don't be surprised if that want $250 to $350 to do so. (Of course, then the phone is yours, and you can sell it to recoup some/most of the cancellation fee.)
On the other hand, if you simply changed to the Simply Everything and decided you didn't use all those minutes, they will let you switch, no problem.
