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.
Immigration. I had to get one to buy my motorbike in BKK. I went to CW immigration office and they did it on the spot. If they mail it to you it takes two weeks and cost 200 baht. Instant service was 1000. I didn’t want to wait.
La Choza Soi 9 Jomtien. Old man weed shop just past the big temple on Pattaya Tai and just past Grand Hotel beach road Soi Wat Boom Jomtien. Candyland and one other Soi Bukhao. And many many more.