Not to rain on your parade here, but I think selling your Sprint HTC EVO 4G and then buying a Sprint HTC Hero and activating it on Sprint's network and expecting them to get rid of the $10 fee per month will constitute a contract breach and thus an early termination fee. I would be very careful about selling your Sprint HTC EVO 4G because you paid a subsidized price for it and Sprint is not going to take you making a profit on that lightly.
As someone stated above, I would definitely go back to a Sprint store and trade in your Sprint HTC EVO 4G for the Sprint HTC Hero within the 30 day limit. That way everything is legitimate and you will not have to fight Sprint over the $10 fee per month and an early termination fee.