Can I switch from a Non-O retirement visa bank method to the monthly deposit method without an affidavit?

Just so we're clear, switching from a yearly extension based retirement using banked money method to one using monthly income by international transfers (when your embassy doesn't issue the affidavit letter) means you actually meet the seasoning requirements of TWO extensions at the same time 😮

You have to follow the requirements of the extension you're currently on (banked money) by keeping the 800K baht in the account for 3 months after the extension takes effect, and then you can't let the balance go below 400K baht the rest of the year

AND

you need to start transferring in to the country a minimum of 65K baht a month, each month, every month for the previous 12 months (meaning you start when you get your extension issued) before you apply for the next extension.

Most immigration offices require you to get a year detail transaction report showing every transaction on your account for the previous year so they can check that you met the seasoning requirements, because as a rule, no one updates their bank book enough to show every transaction)

You might consider keeping 800,000 in your account the two months before to apply for the next extension. If for some reason your attempt to use the monthly method is unsuccessful then as a backup you can continue with the deposit method.

You can remove the monthly 65k tho anytime you want

You did fine explaining it

You can't let the bank balance go below 400K until AFTER you get the next yearly extension using a different means to prove proof of funds (the 12 monthly transfers of 65K baht a month)

That's because the seasoning requirements using the banked money method for the extension you're on now stated you'd bank 800K baht in a thai bank account in your name only for 2 months before you got the extension, you'd keep that 800K baht in the account for 3 months after the extension took effect, and then you wouldn't let the balance go below 400K baht the rest of the year.

So that 400k needs to stay in until you get the next yearly extension granted but once you get that next extension based on monthly income you're free to bring the account right down to nothing.