Don't sign up for Medicare Advantage when you turn 65:

[Ms. Toad]

9. You generally have a one-time opportunity to sign up for a Medigap/Supplement plan.

If you skip it (1) no insurance company is required to issue you a Medigap/Supplement plan and (2) if they choose to issue the plan they can set the fee based on your health/age/etc. at the time you try to switch. (There are a few states which offer a bit more protection - but not many)

So you can drop Medicare Advantage, and rely on basic Medicare plan (which covers 80% of your expenses). You will be on the hook for the other 20% - or - if a company chooses to offer you a Medigap/Supplement plan your costs will almost certainly be significantly higher than if you had made the choice when you started Medicare.

It is not well known, and the government is misleading. You have to dig pretty deeply to find the information I've summarized above - because they imply that you can always switch back to Medicare without expressly stating that you are only legally entitled to the version of Medicare which covers 80% of the costs - not the combination most people think of as traditional Medicare (Medicare + Medigap/Supplement)