How do I transition from a work visa to a retirement visa in Thailand?

Just a bit of additional info. You are going to the Immigration office that your current extension (based on employment) was issued to cancel that one. Then go to the immigration office that serves your address on your TM30 to apply for the new extension based on retirement. Those immigration offices are the same for most people but not always.

okay you're fine (y) there's some "lost in translation" things going on

First you can take the termination paperwork from your employer to the immigration office a week or more ahead of the actual date. They'll cancel that extension and then stamp you in until the date on the termination paperwork.

There is no requirement you show the money came from outside the country because you're not applying for ANY visa inside the country.

You are only changing the reason for your extension, from one based on employment to one based on being over 50 (retirement) and that requires only that you have at least 800K baht in a thai bank account in your name only for 2 months before you apply for the extension.

SO

go cancel your current extension a week or 10 days early, get the documentation together that you need to apply for the yearly extension based on retirement, get the bank letter, update your bank book and go apply for the new extension based on retirement for 1900baht before that cancellation stamp runs out.