I signed up for Social Security when I was living in Thailand and when I turned 62. I used the Social Security office in Manila, Philippines. They also had my address as being in Thailand. A few months before I turned 65 I contacted the Manila SS office to ask them not to sign me up for part B. They said they wouldn’t sign me up unless I specifically asked them to do so. I didn’t ask them but I wonder if they won’t automatically sign you up if they know you are living in a foreign country.
The only difference between the two extensions is the insurance requirement for the OA. You should talk to your insurance company about synchronizing the dates of your insurance policy with the OA extension dates.
Unless you are trying to avoid the insurance requirement you don’t need to switch to a non-O. You can renew your insurance and extend the OA for 1 year inside Thailand.
I was able to get my insurance company to coordinate my policy dates with my visa dates. Instead of trying to get a few days of insurance you should be able to buy 1 year + a few days.
This is one of those things where even if you don’t know the answer you can do with almost no effort and cost. Just do it. Bring it with you. If they ask for it then you have it. If they don’t ask for it they you aren’t out anything.