No worries. The change in the Thai tax law only affects remittances which were earned in a previous tax year. That's why there's no change to pension payments, as these have always been remitted into Thailand in the same year as they were paid. So in theory they've always been "taxable" but because of the myriad of DTAs, expats on "retirement" visas have never been asked to show any documents due to complexity of pension payments and pension funds. It's never been worthwhile the Thai tax office trying to recover anything. They'd be stepping over dollars to pick up dimes, as they say. Nothing will change for pensions, because nothing has changed in the law
Why do I have to convince Thai Revenue of anything? I've never had to do it in all the years I've been here, why would I need to now? I can get a letter from my Superannuation Fund which stipulates my monthly payment if suddenly (out of the blue) they ask for it! 😆
In Bangkok it'll be difficult. When I switched from 800k to 65k/month, they wanted every passbook and statement, and worked it to the nth degree, making sure the 800k had been correctly deposited and that the amount hadn't stopped below the required minimum, and that the 800k had stayed in the account for two months after I'd started the 65k transfers. Pretty intense process, but I'd done it all correctly. Now I'm on the 65k method, it's plain sailing for each extension.
Make sure you have documents supporting your language school and attendance record whenever you enter Thailand, or expect to get interrogated and possibly denied entry