@Kool ******
: I went back to the immigration office at Chaeng Wattana today and, after some lengthy explanations, was given a procedure that I can follow in order to change the type of Visa (without having to wait for the expiry of the existing Visa). I am sharing the information in case it might be useful to someone else on this forum.
This does require travelling out of Thailand and then coming back into Thailand. Here is the procedure:
1. On the day that I plan to travel out of Thailand, go to the immigration office and request that the current Visa be cancelled. They will then cancel the current Visa. As I understand it, it will be cancelled on the passport itself.
2. On re-entry to Thailand, get stamped-in on a 30 day visa exemption.
3. ASAP after re-entry, apply for the non-O 90 day visa based on marriage criteria.
4. After that, proceed as usual in order to get the one year extension to the new 90 day visa.
The important thing in this procedure is that you have to synchronize the cancellation of the existing Visa to be right before you plan to travel out of Thailand. After the Visa is cancelled, you are required to leave Thailand as soon as possible.