Can I change my retirement visa to a marriage visa in Thailand after marrying my Thai partner?

Okay seeing as thailand doesn't recognize same sex marriages (at least as far as anyone getting a visa/extension based on one) let's just go under the premise that their "thai partner" is indeed a female.

As has been pointed out, you haven't been on ANY visa for the last 3 years (if you've been going to the immigration office to get the stamps) you have been on a yearly extension of stay.

Be that as it may, other posters have correctly pointed out that when your current extension is going to expire and you go to apply for new yearly extension you CAN indeed change the reason for that extension from one based on being over 50 (retirement) to one based on being married to a thai.

I would recommend as soon as you come back into the country with your thai wife that you both head over to your immigration office and get the hand out they have outlining the documentation needed to get a yearly extension based on marriage.

You do realize the application for extension will go under consideration for 30 days (and they can do a home visit during that time) before you go back and get the year extension inked into your passport.

Partner can be male or female.

I get confused by marry my thai partner. I know it is discussed in the Government but as far as I know same sex marriage is still not allowed for the Extension of Stay based on being married to a Thai National. Please do not hate me, if I understand you wrong.

I presume you're talking about an extension of stay obtained in-country rather than a visa gotten from an embassy outside thailand

Yes - you can change the reason of having an extension of stay (in your case from >50 to marriage)

As for the documents- I'm not 100% sure but probably best to say which immigration office you're using or visit them and get the checklist or do a group search for extension of stay - marriage.