What steps do I need to take to convert my non-immigrant visa to a retirement visa in Thailand?

this is the stuff you can be asked for when you change the reason and apply for an EXTENSION based on being over 50 (retirement)

I did this at CW last year, suggest to ask your employer HR if they are able to support you with post dated termination letter and the 7 day request before you change the reason of your extension from working to retirement. Some IO ask for tax filings from the employer in addition to two months 800K seasoned for NonO in addition to all the other docs

𝗔𝗹𝘀𝗼 𝗱𝗼𝗲𝘀 𝘁𝗵𝗲 𝟴𝟬𝟬𝗸 𝗻𝗲𝗲𝗱 𝘁𝗼 𝗯𝗲 𝗶𝗻 𝗯𝗮𝗻𝗸 𝗳𝗼𝗿 𝟭𝟮 𝗺𝗼𝗻𝘁𝗵𝘀 𝘄𝗵𝗲𝗻 𝗜 𝗰𝗼𝗻𝘃𝗲𝗿𝘁 𝘃𝗶𝘀𝗮𝘀?

the 800K baht needs to be in a thai bank account in your name only for 2 months before you apply for the year extension.

You aren't converting any VISA to anything.

In all likelihood you don't have any visa, instead you have a stamp that says "extension of stay permitted until" and it has a date and that extension is issued based on employment.

What you would do is get documentation from your employer stating on xxx date you stop work, take that and the proof your work permit is cancelled to the immigration office, cancel your extension based on employment and immediately apply for a new extension based on retirement by having the required documentation for that extension (meaning go to your immigration office and get the hand out listing the requirements for it).

You're not switching to anything just changing the reason for your extension from an extension based on employment to an extension based on being over 50 (retirement). It's just a reason change, and it's done all the time at immigration offices.

That you wanna move to Buriram is great, but cancel your extension based on employment and get your new extension based on retirement before you move. 🙂