Can I switch from a Non-Immigrant O Visa based on retirement to one based on marriage easily?

If you are on a retirement visa you don't need to change it to a marriage visa.

I asked last week in Chiang chan and he just said marry marry marry and hade a good time I have been here since August 2024.

If you enter on a non-O visa, the first extension must be based on the same reason that you got the visa for.

You say "waiting for my retirement visa to run out" so if you actually mean that, the 90 day non-O visa based on retirement, then no, you cannot switch to marriage because the extension must be based on retirement too.

But if you actually mean that you're waiting for your 1-year extension based on retirement (an extension is not a visa), then yes, you can apply for your new extension based on a different reason as long as you meet the requirements.