OTA is over the air, when Sprint pushed out the new update/upgrade to Gingerbread that's how people updated to it. Well if nothings been done to it and it's Gingerbread (2.3 with hboot 2.16) there is no root for it anyways right now.
I did some searching and read a lot of conflicting information on using the Evo on Boost, and what I could gather is you had to either clone the ESN, or find a customer service rep that would add the esn of the Evo to their system, some would do it and others wouldn't, some said it was illegal to do and others said it wasn't, all kinds of different opinions on the issue, I'm not saying it can't be done but sounds like it's a lot of work to do and don't know if it's really worth it.
Here is the link to the thread that I probably got the most information out of, kind of confusing the first page or so but the last page was probably the most helpful, but I would advise reading the whole thread, Good Luck!
Would EVO 4G work on the Boost Mobile Network - xda-developers