You have to keep the 800k there for the first 3 months of the extension and never drop below 400k. You need to show you met these requirements when getting a new extension. The OP is not getting the initial Non-O or a first extension but a new extension. ;)
how can he get a bank letter if he doesn't know the branch and, more importantly, the 800k was not seasoned and maintained to meet the requirements of the first extension?