I will be getting married in Thailand next year. I will stay for 6 months on a double entry Tourist visa. When I come back I should apply at a Thai consulate in the USA for a 12 month multiple entry Immigrant "0" visa based on marriage. Once back in Thailand I can change that VISA to a marriage VISA at an appropriate immigration office. Is that correct ?

There is not such thing as a marriage visa. There is the non immigrant O visa which is issued for those with a Thai spouse and there are extensions of stay for marriage that are issued by Immigration if you meet the requirements of 400k in bank account two months or 40k income per month by Embassy letter.

Most Consulates prefer to issue single entry non immigrant O visas expecting that you will extend your stay at Immigration.

Ok I understand--Thank you. Extensions for stay on "O" visa which is based on marriage are issued at Thailand immigration once you meet the 400,000 or 40,000 requirements. The initial immigrant '0" visa based on marriage to a Thai is obtained from your home country Thai consulate.

It does not have to be home country - any Royal Thai Consulate can issue such a visa; and most will.

Yes. You will be extending your Permission to Stay, not your Visa.

The Non Imm O Visa can be issued in countries other than your home country if you meet their requirements.

You don't need to apply for a 12 month multiple entry Non-Immigrant "O" visa.

A single entry visa will be sufficient and save you a few dollars.