What financial documentation is required to switch from an O retirement visa with 800,000 THB to the 65,000 THB monthly income version in Thailand?

In order to switch from banked money method to monthly income method for proof of funds you need to do two things.

First is keep the seasoning terms you agreed to on the current extension you're on based on bank money method. That means you keep 800k in the bank for 3 months after your extension was granted and then never let the balance go below 400k the entire rest of the year until you apply for your following extension.

And while you're doing that.

You need to transfer in a minimum of 65k per month into a thai bank account in your name only from abroad, each month, every month, for the previous 12 months before you apply for your next extension.

You need to have 12 months of transaction detail report and all the transfers need to code as international origin.

You cannot get a yearly extension of State based on monthly income method without 12 months of transfers.