I have a multiple entry, Non-immigrant "B" visa issued at a consulate in the USA. The validity for new entry expired yesterday, but my current entry is valid until December 7. I have no work permit. My visa was issued on a statement of a partnership in which I was an agent, buying paintings in thailand to be sold by my partner in the USA. I was told, maybe inacuratly, that I didn't need a work permit for this.

I would like to change from the "B" to a retirement "O"

My question is; can I go to Thai Immigration at Suan Plu with the required documents and apply for a "Retirement O" directly or am I going to have to leave the country, get a non-immigrant "O" and then come back and apply for the Retirement "O".

Thanks

You already have a non immigrant visa entry so you should be able to just present the required documents for retirement and obtain the extension of stay.

Does he need to do that at least 21-days before 07 Dec? Or can he do it any time up to 07 Dec?

Also, will he be violating the terms of the retirement extension if he is buying paintings in Thailand with intent to resell via his USA partner?

The 21 day prior is the old rule for change from a tourist visa to a non immigrant visa at immigration. He is not doing that and can go anytime up to expiration of his permitted to stay stamp - but I would suggest about a week early just in case anything extra is required.

I am not sure he is still working but any work requires a work permit and they are very rare among retirement extension of stay holders (although there were several recent reports that it has happened).

can go anytime up to expiration of his permitted to stay stamp -

What happened to the 'last 30 days of your 90-day permitted-to-stay' period? Has this gone by the wayside? Or is this only for support extensions?

The last 30 day is often stated for marriage extensions of stay (where Bangkok makes the decisions) but for retirement extensions they seem to be more liberal and will accept earlier if you have the paperwork ready.