What happens to my retirement visa extension if I leave Thailand before the three-month period after the fund verification?

There are lot of mixed reports here. Illustrates the need to verify with your local immigration office. Last year, upon returning to immigration to pick up my passport with new Extension of Stay, the officer handed me another form, and told me to return on the date stamped on the form (90 days out) with copies of my updated bank book (updated that day). This year, they didn't give me the form. I asked the officer about it, and he said that since I kept the funds in the account for the whole of last year, they didn't need me to come back to verify again in 90 days. He said he would see me again next year. Friendly guy.😉👍

I do this every year, and when I apply for my next annual extension, my Immigration accepts my monthly updated bankbook as proof I did not violate the upkeep requirements for the deposit. It depends, some Immigrations offices make up their own rules

You are a little confused, the 3 month check will happen when you go to do your next yearly extension. Once you are 3 months into your yearly extension there is no requirement to go to an immigration office to prove funds.

Where you are is immaterial to having the funds in the bank, as long as you dont take the money out.

Don't forget after the 3 months you must maintain it at 400kthb until bringing it back up to 800kthb for your next extension

nothing special happens. you can provide the documents about your bank account as long as your bank account is “alive”, regardless you are in Thailand or out of the kingdom. and as you should be able to provide documents or passbook to prove your bank account balance for three months including time you are traveling out of Thailand, you just provide it if you are asked.