No that is not true, some immigration offices will require the 400,000 baht to remain in your account during the under consideration period, but after the stamp is in your passport you are free to take it all out. Just make sure it is back in your account 2 months prior to your next extension.
absolutely correct John, more and more offices will not except the income method unless you are actually working in Thailand, where we are in Cha-am Phetchaburi immigration will only except the banked money and I did my extension last month and for the first time they asked for a 12 month statement, for years they only required a 3 month statement.