Picture your HSA as an employee who has been doing the same job for twenty years.
Collecting deposits. Investing them. Quietly growing in the background while you ignored it.
The day your Medicare coverage starts, that employee gets reassigned. Same person, brand new job description.
No more deposits. Ever. But now it is authorized to spend on things it could never touch before, including the Medicare premiums coming out of your Social Security check right now.
That handoff catches people off guard, mostly because nobody explains it. You get a Medicare card in the mail and a welcome packet about Part B, and not one page mentions the account you have been feeding since 2009.
So here is the full handoff. What stops, what starts, what stays exactly the same, and the one date you need to write down before anything else.