It can be done, and it's cheap.
It's hard to discuss on forums because the details can get you banned.
While we know about the possible (junk) 600mhz ZTE phones coming, Virgin is expected to announce some new 4g phones in the next 4-10 months (Yes, 4g on Virgin). The first is expected to have a faster processor than the Evo and Triumph and similar screen size. It's also possible we could see the new HTC One V.
You may want to wait rather than go down the Evo road.
On the other hand, if you still want to, keep in mind that it...
Violates Virgin's terms of service. You could lose your account.
There is no support.
You risk bricking both phones.
It takes a lot of tedious work (15 hours min,. but most likely closer to 20+)
MMS is sketchy in many cases.
You will need to download pirated programs, some of which maybe hard to find, unless you want to pay $800. In some states it is flat out illegal.
In others, and federally, it is a slightly gray area, though odds are no one will come knocking on your door over it.