Yes - if you stop - you will lose functionality....
Thats the polite, medical answer.... The stark reality is SOME of us POTSies CANT get out and excercise daily because it is more dangerous to do so than not - a fall could break something, the energy levels required to stand up alone are more than some of us have and our heart rates, chest pains, BPs, and the dizziness just wont take it.
So - my advice is as most will have been told - keep your feet and legs moving even if you are still laying in bed - rolling the ankles - raising the legs a little if you can etc.....
If you can - get a yoga band or two and use those to stretch your arms and legs and keep things moving up and down and if you can lay on your side - then side ways as well depending on what else you have going on with hips and back etc..... stretch your arms out etc and back muscles ...
IF you can - light excersise - walking - even if its using a walking frame, swimming or walking in a swimming pool, some POTS patients can run, jog, go cross country, do weights, play sports like football, netball, basketball etc - personally - I would die if I attempted ANY of that. .... Stick to what your body allows and push it a little every now and then to see what you can handle but be prepared to be totally exhausted for days afterwards and incapacitated depending on the severity of your POTS.... again - doctors and physios recommendations for YOUR situation.