Do I need to keep 800k in my account for 3 months to switch from Non-O to Retirement visa in Thailand?

You're not converting anything visa wise 😮

I'd bet dollars to durian you don't have any valid visa 😉 but instead are here on an "extension of stay" based on being the spouse of a foreigner on a legal visa/extension.

All you're doing is a REASON CHANGE from one extension to another

The seasoning requirement on banked money method for extensions of stays based on retirement are "the funds need to be in a thai bank account in your name only for 2 months before you apply for the extension, then the 800K stays in the account for 3 months after the extension is granted and after that the balance can't go below 400K baht the rest of the year

You would get everything ready for the retirement extension (all the documents, bank letter, statement, updated bank book, map, copies, lease TM30, etc) go in cancel your extension based on being a trailing spouse (you might or might not need the primary visa/extension holder with you to cancel the current extension you're on as it's just a piggy back one)

As soon as you cancel the current extension you'd apply for a new yearly extension based on retirement for 1900baht

GO to your immigration office, get the hand out listing the requirements for the extension based on retirement well before you're going to try this change the reason for an extension.