How can I change my Non-OA (Retirement) Visa to a Non-O (Marriage) Visa in Thailand, and are there any time restrictions?

What you were told is correct and I'll explain the reason, and your options.

You're currently on a retirement VISA. You cannot change from one visa to another. You can only change from one EXTENSION to another. This means you would need to get a 1-year extension based on retirement before you could switch the reason the next year to 1-year extension based on marriage.

The easiest thing for you to do would be to leave Thailand without a re-entry permit after your OA visa expires. Then either apply for a 90 day non-O visa based on marriage at a Thai embassy outside of Thailand, or return as a tourist and apply to convert your tourist entry to a 90-day non-O visa at your local immigration office.

I had an OA in Bangkok for 5 years as retired. If you are at the end of the first year of your OA then I believe that your first 1 year extension has to be the same as your original OA and that means you have to extend as retired for the first extension. The TM7 form for a 1 year extension has an entry at the bottom to enter the reason for the extension. This is where you enter "retired". I think for the second 1 year extension you can enter "marriage" on your OA visa. I also believe that even if you are on an OA visa, if you extend as married, you no longer need to have the required health insurance. Contact Tod Daniels about this.

If you don't want to go another year as retired then you will have to leave and re-enter to start the process for an O visa and subsequent 1 year extension.

Also, just because you got married it doesn't mean you have to extend as married. I continued to extend as retired after I married because the process and documentation is easier. You do have to meet the higher financial requirement for retirees though.

the requirements for the "change of visa type" can be seen on this webpage of the Thai Immigration. Click onto number 6 to open the pdf. list of requirements for the "change" to the 90-days Non-Imm-O marriage visa

In order to change to a Non-Imm-O family visa, you first must invalidate your existing stay permit (i.e. the Non-Imm-O/A visa)

A Non-Imm-O/A visa cannot be made invalid in the first year, because it is a multi-entry visa type.

You will need to await the date after the visa validity has expired.

After this date you won’t have a re-entry permit any more, which means that when you exit Thailand, your previous stay permit will become invalid, and the visa will also have expired

Only now the path is free to apply for another visa.

Only then, you can re-enter Thailand visa-exempt, visit Immigration ASAP and ask for the “change of visa type” from the 30-days exempt-entry.

You will need to show proof of your marriage, Immigration does not accept foreign marriage documents, they only accept a freshly printed marriage registry document, the “Kor Ror 22” from the local Amphoe office.

You will need to show 400.000.- THB in a bank account in your sole name. No seasoning required at this point.

After having been granted the 90-days Non-Imm-O family visa, you need to wait for the deposit to season for 2 months, before you apply for the “one year extension of stay based on being married to a Thai wife”

You cannot change from a Non-OA to a Non-O, you have to re-enter when your initial Non-OA has expired or re-enter on an extension without a re-entry permit. You can either re-enter with your Non-O from a neighbouring Embassy/Consulate or re-enter as a tourist and start the Non-O in Thailand (assuming you already have a Thai bank account and the 400k deposited).