The i9 is an iden phone, so it can be used on any iden carrier (when unlocked). If trends stay the same, no software mod NEEDS to be done to use a Sprint (Nextel) phone on boost. Should be able to just pop in the SIM and go.
You are correct ddwithoutfear. As long as the phone comes from Sprint (Nextel) then you could just pop in a Boost Mobile sim card and go, now if the phone is from Boost Mobile and you want to put a Nextel sim card in it then it does have to be unlocked.