@Richard ****
I still think you're way better off doing it all where the current extension was issued rather than cancelling the current employment extension and then going to C/W wanting to get a new year extension based on retirement.
So GO talk to your immigration office (where you live/got the extension/do 90 day reports) and see what they say about cancelling your current employment extension a couple weeks early then immediately applying for the year extension based on retirement
As Gary pointed out you will have to have the 800K baht funds seasoned in a thai bank account in your name only for 2 months before you apply for the yearly extension. You'll need the 'standard' bank letter, the 3 month printed detail transaction report (to show the 800k was in the whole 2 months before you apply). You have to update your bank book the date you apply for the extension along with providing them all the documentation that office typically requires for a year retirement extension