you can ONLY change the Virgin Mobile service to another Virgin Mobile phone.
if you want a phone that is not on virgin mobile, you have to go buy that phone on that phones carrier, and ask them how to port your virgin mobile number to it.
honestly changing numbers is no big deal, and much less hassle. just text your contacts the new number when you get it. simple.
of course, no other carrier has the phone AND the plans to compete with VMusa, but if money is no object, get whatever you want. Galaxy Nexus maybe?