Pick either way.

Remover the motor and then remove the transmission, or lower the subframe and just remove the transmission out the bottom while supporting the motor from the top.

And do you want to clean up 12 quarts of fluid off the ground or just a little spill from it seeping out from places??

You'll want to drain the fluid and you'd need new fluid for the other transmission you are installing, you'd be foolish to not replace the fluid.