Can I switch from a retirement visa to a marriage visa in Thailand, and what are the implications of leaving shortly after receiving a retirement visa?

Yes, the next year you can apply for your extension based on marriage. But even normally it's way more to do and I've heard many stories that immigration 'seem' to frown on this and make it especially difficult, giving you a huge hassle...

My friend had the retirement visa for many years and recently switched it to a marriage visa by getting married to his thai gf after 25 years together. In essence as a pensioner requiring only 400k baht but the process is more involved and vetted for potential scam marriage. Multi entry permit needed with the retirement visa to come and go as needed per your question. Also alot depends on whether you have a thai bank account already and residence. Cant you just forgo the retirement visa and apply for the marriage visa ? Get an agent to assist you …the marriage visa is more involved literally 🫣☺️

If you obtain a 90-day non-O visa based on retirement, you can only apply for the 1-year extension based on the same reason the visa was issued, so it would have to be a retirement extension. The following year you could apply for the extension based on marriage instead of retirement though.

And if you receive or enter with a 90 day non-O visa in January, your permission to stay stamp will run until some time in April. That means if you wanted to keep that and apply for the extension, you would need to be back before that time ran out to apply for the extension. May obviously wouldn't work, and April would only work if you're back early enough depending on when your permission to stay started.

You would also need to purchase a re-entry permit before leaving so your stamp isn't cancelled by leaving.