Never heard of that. Why not post the agency name and see if anyone else has dealt with them? Never sign blank paper it's like signing a blank cheque .
If you have to do something then write a letter to the embassy and sign at the bottom. I'd post the agency name here first to be safe though.
SCB used to let you change your limit for a one time withdrawal by putting your passport number in. Might just be for customers that bank with them. It offered me upto 100k
Apply for the one month extension if you want to stay another 30 days after the amnesty finishes on 26th Sept. You can apply for that extension from the 1st August if you can get a letter from embassy / consulate.