Family Plans and Used Phones

My family has a plan on Sprint encompassing 3 people. My first phone was added Dec, 2009. It eventually needed to be replaced, and so I got another phone earlier this month, and it too already needs to be replaced. If I buy a used phone on eBay with a clear ESN, will it be easy to deactivate my current broken phone and activate the used phone, with no hassles?

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If the phone is compatable with the Sprint Network, The transfer should not be a problem

So since it hasn't even been a month since the purchase of the last phone, will there be any additional fees? Perhaps having to pay the bill twice that specific month?