I "converted" my long time marriage O visa to a retirement O visa a couple of years ago. My local office has not changed the requirement to 800k, still 400k. I guess, like everything else with visas, it depends on the individual interpretation of your local Immigration office.
Surely it would be easy enough to contact one of your “Thailand people” who would have a record of your number? You should be able to top it up once you have the number.