I have a fixed account. It’s just a savings account with different rules about how long the money has to stay to get your interest payment. Also (if I remember correctly) minimum deposit is 2000 baht. I found this irrelevant because I made one deposit to meet my visa requirements and then don’t touch it at all. Although you are required to show a transaction prior to applying for your year extension. I simply do an internet transfer in of 2000 baht and take it back out the day I get my letters.
I did exactly what you want to do. I waited until renewal time for my extension and simply changed the reason from “Marriage to a Thai” to “retirement”. Easy as can be.