I am glad you have as much information - you have done well.
It is against MC law for an agent to sell you a Medigap policy is you ARE enrolled in a Advantage Plan. The reason for this is that Medigap bridges the gaps in Original Medicare. If you are enrolled in an advantage plan, you are not actually on Original MC. Then someone has sold you a product that is of no use to you. Medicare prohibits that.
OK, then lets go further and talk about Can you Switch to or from original MC. Yes you can do that. You are correct. To drop an Advantage plan during open enrollment Oct 15 - Dec 7, simply enroll in a Part D drug Plan and your Advantage Plan will be canceled by MC on Jan 1 of the following year. You will then be on Original Medicare. BUT, that DOS NOT entitle you to purchase a Medigap policy after your IEP if you do not have an SEP Special Enrollment Period.
On the other hand, if you are on Org MC and want to be on an Advantage plan, then simply sign up for the Advantage plan during Oct 15 - Dec 7. Jan 1 you will be on the Advantage plan you selected, MC will cancel your Part D drug Plan.
About SEP. These are exceptions that permit beneficiaries to sign up for various MC products. Without your IEP or an SEP, you will not be permitted to sign up for Medigap.
Let me give you a SEP example. Many people (and a lot of agents) don't know about this SEP. Normally you can only sign up for Medigap during IEP. I can only speak for MO, some states are different. In MO, if you signed up for an Advantage plan during your IEP, and had the plan for less that 12 months, you may have an agent drop the Advantage plan and sell you one of certain Medigap policies. Plan N is excluded from this SEP - it cannot be select then. This gives you an opportunity to use the Advantage plan and if you are dissatified, you may use this SEP to drop the Adv pl and purchase a Medigap on Org MC.
SEP exceptions are needed - for example, if a working person over 65 chooses (Working Aged MC term) to delay enrolling in MC, he/she will be provided a short (usually 30 days) SEP to enroll in Medicare without penality, including Medigap.
This statement made by James k0ua is pretty clear and correct.
"You CANNOT and NEVER WILL be able to buy a Medigap (supplement) policy for the rest of your miserable life."
You must purchase Medigap during IEP or using an exception SEP. Period.
You can switch from Org MC and Advantage every year if you wish. BUT not Medigap. You have limited purchase opportunities.
I am not giving you advice. But as I said before, it is MC law. A lot of people use and like Advantage plans. I had one the first 10 months when I went on MC. It was fine. It may be a good fit for you.
But Original MC may also be a good fit. Those are options for you to consider.
When I was counseling, many people (including my own father-in-law) complained - "Why does MC make it so complicated?' The answer - Because we are being offered a choice about our health care and costs.
Many times I was asked - What do you think is best for me? I would NEVER answer that. It is an individual choice.
This is a hard subject to go much deeper here. I could write you a book. Best Wishes, Larry