After changing the logic board, you will need to activate the "new" phone. You can do this by logging on to Sprints website, and entering the information (ESN/MEID/Etc...) of the new phone. You shouldn't have to worry about any new fees, since you're swapping identical phones. If you have any trouble, you could always take it to a Sprint store and have them activate it there, just make sure you have the ESN/MEID written down somewhere to show them. This also won't reset your contract, as I replaced my HTC Hero with an Evo 9 months ago using this method, and just got an email from Sprint telling me that I'm eligible for a 2-year upgrade.