The respective vendors say that it needs to be jailbroken to do so isn't that just to use non apple software? I understand that Sprint is the parent company of Boost or something like that.
No, it doesn't need to be jailbroken... First - deal with Sprint - you've completed your contract with them, they are now obligated by law to remove the carrier lock on your device... once that is complete - you deal with Boost... it should be as simple as giving Boost the MEID, and installing a Boost SIM for their LTE.