How can I change my working visa extension to a marriage-based extension in Thailand?

Okay I think what has eluded ALL of us (or at least me 😛 ) is that the O/P is a FEMALE foreigner married to thai national, and in that case there is no proof of 400K baht for the Non-O visa in country OR the year extension. <- the must be married 2 month rule still applies though (because that is immigration office dependent)

They should GO to the thai consulate in Savannakhet Lao and get the 90 day Non-O visa based on marriage, come back get stamped in for 90 days, wait 2 months and then go apply for the yearly extension at the immigration office in Bangkok

As a simple answer, you cannot apply for your extension based on being married to a Thai as you planned. You have to leave Thailand and enter Thailand on tourist status or Non-O based on marriage to a Thai from the embassy/consulate outside Thailand.

This is because Bangkok doesn't allow you to apply for a visa/extension based on being married to a Thai national unless you have been married for a minimum of 2 months.

Just as information in case you have been married for more than 2 months as of application day for marriage visa/extension.

Technically you have to apply for cancellation on the last day and you need to apply for a new extension on the same day not to overstay.

But generally, you may have a 'margin' until the application of a new extension

- at most immigration offices let you apply for cancellation of an extension several days earlier

- as BOI/IEAT employee, you MIGHT be granted 0,7,14,21 days grace period to prepare to leave or apply for another extension. You need to check with OSOS how many days you could get.