Trying to figure out brake problem with rear disc upgrade

[Hydratech®]

I suggest you swap the calipers and live happily ever after with the bleeders up top where they are supposed to be.

What the previous owner may have done to get it to work this way: While absolutely most certainly not the right way to do it, you *could* yank the calipers loose while leaving the brake lines connected, put a some appropriately sized hardwood inside of the pads to take the place of the rotors, orient the bleeder screws up top, and then bleed the air out the brake system. Once bled, you could then re-install the calipers and take the vehicle out for a test drive. If you feel that all is well at that point, then you could swap out the calipers right / left at any point, though I do suggest you get it done right now...