relative in Va (state) 73 with pre-existing conditions, switching from advantage plan to suppolement plan, confused about timing of when to switch

can anyone offer any insight, the more i read about fall and spring enrollment, the less convinced i am i have a handle on it. She is concerned at present about needing to travel out of state for care, her current advantage plan would not cover it, she thankfully could consider a supplement plan that she'd need to pay for monthly, the question is..

when does this have to be done, and which steps when?

she's in touch with an insurance salesperson that i feel is confusing her further. On december 7th fall enrollment ends. should she drop the advantage plan tomorrow and go on regular medicare, then asap enroll in whichever supplement plan she can get underwritten? is there some reason she needs to wait until january open enrollment?

I'm sorry, i'm sure this question is often asked but with only one day left I thought I'd try asking you guys instead of another two hours of googling. tomorrow i will call the navigator locally..