How soon can I withdraw 400,000 THB after renewing my retirement visa in Thailand?

Are you starting to do the 12 65k montly international transfers for your extension next year? That is how you free-up the 400k for withdrawal (and the 65k can be withdrawn immediately after deposit).

The rule is you must have the 800,000 in the bank for 2 full months before you apply for your extension (not before your extension ends, but when you apply) and then it must stay at 800,000 for 3 full months after you receive your extension. Then it can never drop below 400,000 the rest of the year. You bring it back up to 800,000 next year 2 full months before you apply and repeat the cycle.

As part of your application next year they'll want to see a 12 month bank statement to see that you kept it at 800,000 for 3 months after last year's extension and then never let it drop below 400,000.

800k 2 months prior to application and 3 months after. Then can drop to no less than 400k and has to back up to 800 2 months prior to next application.