Well, I got my "donor" Crimson HISS in the mail today and went to work.
I think it turned out pretty well -- I like the overall look of the Retaliation HISS body better, but prefer the tracks from the PoC version - mashed them together easily enough.
So here's my exclusive CHSDCC (Chief Hates SDCC) Crossover Soundwave!

Basically, you take the little blue inserts from the Retaliation treads and graft them onto the PoC treads. The diameters of the round parts are identical, but they don't line up perfectly without mods (which is why you can't just swap out the chassis). I ended up chopping mine down quite a bit so that the wheel base wouldn't be so wide.


They have to be glued in place, so basically you're left with (even more) useless Retaliation treads. The PoC HISS ends up being track-less as a result, but that was my intention anyway - I'll paint up the PoC body and make it a HYDRA gunship or whatever.
So even though it's do-able (took only about 10 minutes -- would have been less, but I decided to saw down the inserts), you're basically bashing two HISSes together and only getting one viable vehicle out of the mix. I wouldn't suggest trying to build a whole tank battalion using this method - but I only wanted the one, so it was worth it, IMO.