One thing you do have to remember is that in Thailand there is no one rule fits all. Example, the requirements for extensions are specific to each immigration office. So you can ask a question and get different answers which are all correct but specific to an individual office. The flippant remarks posted by some with nothing better to do aren't difficult to filter out.
The 800k has to remain in your account for 3 months after the extension is granted and be back in 2 months before you apply for your next extension. Can never go below 400k for remaining 7 months.
I'm not 65 until January and I've spoken to International Services on at least two occasions and they've been extremely helpful, I guess it just depends on who you get. But they are right, you can't apply until you have your pension. There will be an application form to complete, they'll require proof that your moving (tickets, address etc), they'll also require which bank you want the pension paid into (you can elect a NZ one or Thai one). You will also be required to have an interview which they told me can be arranged by phone in many cases. Hope this helps and clarifies how it works.