Can I switch my parent from a Medicare Advantage plan to Original Medicare with a Medigap supplement?

My mom's physical therapist has asked me to switch her from a Medicare Advantage plan to original Medicare. Her reason is that because my mom has Parkinson's, she would benefit greatly from regular, ongoing physical therapy, and Original Medicare covers that.

The physical therapist says right now, my mom has to be seen by a doctor and get authorization for physical therapy, and the therapy only lasts for a set amount of time (like six weeks). She said it took a week to get it all authorized last time. She says this is different than how it works with Original Medicare. With Original Medicare, my mom can get covered, ongoing PT without needing to re-authorize.

I think she is annoyed at having to do the extra work, but also she wants my mom to benefit from ongoing PT, as do I. She says we can also pay out of pocket for her services ($50/session after partial insurance coverage), which we can't afford more than a few times each month.

I emailed a local agent who said this:

"Since she has had her current plan for several years she would not be able to switch back to a Medicare Supplement and Prescription drug plan unless she moved to another county where her plan is not portable. It does not sound like she will be moving. So the only way she can change plans now she gets Extra Help (the Low-Income Subsidy) through Social Security or she is dual eligible and has Medicare and Medicaid."

So I assumed it was a lost cause and told my mom's physical therapist, but the physical therapist is pushing me to contact Medicare directly to ask because she thinks it's possible. I think maybe she's had people just switch back to original Medicare, no MediGap, which sounds scary to me? I'm assuming that my mom would be denied MediGap.

Is it worth it all to call Medicare about this, or should I just let it go? Is it unreasonable to ask the physical therapist to just keep renewing my mom's PT?