Can I withdraw my 800,000B deposit after my recent visa renewal and switch to the monthly income method for future renewals?

Don't forget to consider the Thai Revenue Department Tax implications on money remitted into Thailand if you live in Thailand for 180 days or more per calendar year. If you already have the 800k banked, it won't be assessable for Thai taxpayers going forward. Remitted money is most likely assessable for tax purposes in the UK and Thailand.

Got this after my extension granted

Todd gave me this advice 9 months ago and he is absolutely right. In addition, I am keeping the 0 baht in the bank until I get an extension based on monthly income just in case something goes wrong or my immigration office change their minds.

If you used banked money method to get the extension you got 2 weeks ago you are NOT able to withdraw the funds until 3 months after that extension.

It doesn't matter if you have been meeting the requirements for proof of funds by monthly income or not because you used banked money method to meet the requirements for the extension you're on now.

That means you have to meet the seasoning requirements for banked money method for the whole year on this extension; 800K baht in a thai account for 3 months after the extension is granted and then never let the balance go below 400k baht the rest of the year

AND

if you're wanting to use monthly income NEXT extension you'd also have to transfer in a minimum of 65k baht a month, each month, every month for the previous 12 months before you apply for the extension.

In essense to switch from one proof of funds to another (banked money to monthly income) you meet TWO sets of financial requirements; one for the extension you're on and one for the extension you will apply for next year