Exstension of stay valid until end of August. Moving to hua hin end of June. Will close bank account with the required 800000 bht the day before leaving. Arrive Hua hin obtain TM30 open new Bank account deposit 800000bht. When go to renew my exstension of stay  in August will Immigtation accept that for a few days or a week at most the required 800000bht was not taken out to be used . Thanks ubon joe and jack in advance.

I would not do that OP. 

It appears you would have the 800k baht in the bank for 2 months so that part would not be a problem.

The problem would be the having the 400k baht in the bank that is required.

I suggest you not close your existing bank account. Open the new account and then transfer the 800k baht to the new one.

As Ubon Joe said, the 400k has to be there or you negate your extension. I'd take 400k and open the new account THEN transfer all the rest from the old to the new account and let the old account die of malnourishment. (bank will close it for no funds or use over time)

If you don't do the money swap in two steps it could cause you to not meet the requirements for seasoning of funds using banked money method

I would recommend you move 400K baht to the new bank account FIRST.  Then a few days later move the rest.

That way you meet the seasoning requirements of having a minimum of 400K baht in a thai bank account in your name only during the year. And as long as the 800K baht is in an account for 2 months before you apply for your next extension you should be okay,

Keep in mind there is a good (in fact great) chance that you will be required to have the year transaction history from the bank account you have now (to show the officer you kept the 800K baht in the bank for 3 months after you applied for the last extension AND that the balance didn't go below 400K baht the rest of the year.

I am near 100% certain they will NOT accept your application for extension if the money is "missing a few days" <- as in not in one account or the other when you go to apply for your extension. The recommendation I made is just to stop you from running yourself off the rails