There are different grounding requirements for a main panel -vs- a
subpanel. So I don't know how you would get around that issue without a
permanent re-wiring job.

Panel require grounds and subpanels require a ground wire back to the panel.
No big deal to disconnect the ground when changing them to subpanels.

Beyond that, the project is way over my head. Seems silly to approach it
without bringing in an electrician who knows what he is doing.