YOu will be charged $10.00 a month no matter what. It is considered a 3G smartphone and as soon as you switch to one, or even if you were to swap your phone with another Optimus, as soon as Sprint's system sees the activation, you get dinged $10.00 a month.
I have a couple of friends that have EVO 4G on Sero plan, its just gonna be $10 extra every month but you will have to call in to their employee dept to have them change the plan. CSR reps are able to do it but you might have to call a few times until you get someone with the knowledge of that plan.
There is a $10/mo fee (on top of the $30/mo for SERO) for any 'new' smartphone (ie, BBerry, Android-- basically anything except WinMo Treo/TP2 etc). If you want 4G you will have to pay an *additional* $10/mo (for a total monthly bill of $50/mo).