You would need to cut a lots of trunk floor out. And even then the hv-battery might need to be modified to fit in there. And then you would need to build new air ducting to get the cooling to work and extend the power cables (and all the other wiring) to reach the new location. Also in the new location hv-battery would get water damaged much easier as it would be at the lowest point of the trunk. If any water would get in the trunk it would go directly to hv-battery.
Big big job!! I see the downsides that @valde3 observed, plus some more potential ones like rerouting the exhaust and other stuff. I don't see the upside? What's the purpose of taking on such major surgery?
One reason would be if you where using Prius as a cargo vehicle you would get a level floor at much lower height. If you remove the rear seat and cargo box only thing that's stopping you from building a cargo floor at that level is the hv-battery.
I'd think it would be easier to replace the hatch with a custom-made bubble of some sort to gain more height over the existing deck. Granted it's not going to be easy in terms of structure, glass, security etc but I think it's a few less problems than a battery relocation. More do-able for the DIYer.
That would only get you the added room for trunk area. If you would relocate the battery you would get the high area with flat floor all the way to the front seats (as you would be removing rear seats). If one seat is enough relocating the battery to passenger seat area would probably be the easiest as that would require smallest amount of fabricating. At least that would be a lot easier than a building a good custom hatch. But anyways that was just my ques on why OP was thinking about this.
Thanks for your thoughts. The goal would be for increased cargo space. I've been using my gen 2 for quite a bit of camping and traveling. Nights that I spend in the prius have been very comfortable. Increasing that comfort would go up drastically with just a few more inches of head room by lowering the bed space. I can see this job is probably out of my ability. thanks you guys for your input
Yeah, that sounds like a good motive. But it's a huge job. Probably not impossible, but that extra space would have to be something you really really want.