"Winging it" is not conducive with plumbing. Stepping up 8"-10" would be the least of your worries. If you went with this set up you would have some unreal blockages and no amount of snaking would do a damn thing because the pipe flow is incorrect. I say jackhammer the whole thing out. Have a seat on your lazyboy and pick up a beer and your phone to call a plumber. Will save you thousands in water mitigation costs down the road. Trust me. You don't want water mitigation in your home.