you should be able to if they have the 30 day trial period as Verizon does. You may also have to pay the difference of the two phones to upgrade. In any case it's worth going in and speaking to them directly to get the phone you want.
Does anyone have an idea of when the 8350 will drop on Sprint? I need to get wifi and I don't want to go Android. Essex is just too far out. I will add a second line and then upgrade my primary when Essex does drop.