Depends ENTIRELY on your immigration office in Thailand. They are the only people who can give the correct advice on this one, as each office has different approaches to the 65k bank transfers. I switched from OA (which did not require the 800k) via an out/in on exempt and then 90-day non-O from CW, then 12-month extension. I had started the 65k transfers whilst on OA, so I never had the 800k lump sum. However, I also never used an agent
"They may not want to do" - but that's their choice isn't it? Depends how keen they are to get a longer term visa, or whether they want to keep milking the visa exempt/extension/border scamper, and then complain when they're questioned and potentially denied. Comes down to choice