How can I protect my foundation when removing old retaining walls and regrading?
Let's address the safety issues first...
Working under your deck and porch obviously presents serious safety concerns. A wrong move here could be deadly. I'd follow these guidelines and good sense in general:

Block access to the deck during all work. No one should be allowed on it.
Work one footing (or maybe a few) at a time, relying on existing (and eventually replacement) posts for ongoing stability.
Excavate around the footing to the new grade, then set up temporary posts. These should be on solid bases, which can simply be crosswise lumber, to prevent settling if it rains. Use multiple posts, with one on each side of the deck. Be sure to leave adequate space to dig the new footing in below grade. Posts should be equal to the task. Two 2x4 boards in a well-fastened T configuration are ok, for example. A single 2x4 is not adequately stiff. Posts should be screwed to the beam at the top to prevent accidental displacement.
Once the deck is securely supported, remove and replace the original post and footing. Before cutting the post to length, evaluate the overall level of the deck and make any adjustments.

Then, about your foundation...
Barring any odd, unknown issues, your foundation does not depend on backfill for stability. It's stable with nothing around it. In fact, more problems occur during backfill and as the soil settles if it's not done properly.
However, your home may be taking advantage of the insulation and ground-source heat properties of the existing grade. Consider whether your energy walls are adequate to handle the new exposure to the elements lower on the wall.
I would advise you to work slowly and carefully and regularly check inside the foundation for signs of movement or other effects. Otherwise you should be fine.