Can I switch from a work permit to a retirement visa while in Thailand or do I need to apply from Malaysia?

You are not actually switching to any Non-O anything, you'd be cancelling the extension you're on based on employment and then applying for a new extension based on retirement (being over 50)

You're just changing the reason for your extension (and that's done all the time)

Your work permit has nothing to do with you being able to stay in thailand 😮 That is what allows you to work here..

It's the "extension of stay" stamp in your passport that lets you stay here <- and right now that extension is based on employment but you want to change the reason to based on retirement

First thing you would do is, make sure you meet the financial requirements; have to have 800K baht in a thai bank account in your name only for 2 months, <- that is going to be critical to switching the reason for your extension from employment to one based on retirement

Then you would get documentation from your employer stating on xxx date you stop working.

You go to the immigration office a week or so before that date, cancel the extension of stay based on employment, and they'll stamp you in until the termination date on the paperwork

Then some time before that stamp runs out you'd get all the documentation required to apply for the year extension based on retirement; bank book, bank letter, bank statement for 2 months, map, TM30, lease, etc (everything the office requires), show up and apply for a year extension based on retirement for 1900baht

In face you can cancel your current extension based on employment and immediately apply for a new yearly extension right then based on retirement if you have all the documents ready to go at that time

They will issue the year extension on the spot and after that you'd buy a re-entry permit if you want to exit/re-enter the country during the year your extension is valid for

That's how you go from an extension based on employment to an extension based on retirement.