Anything signed on Tuesday before first break is considered pre-approved for the following week. For example if you signed up for flex down the following Friday and they call either pre-approved for the week OR a daily flex down call, you aren't required to come in.
Now for changing your flexing preference after the call has been made...it's pretty much up to your GL. Usually if you had signed flex down but want to actually stay they will accommodate you since more than likely someone else will just flex home instead. Overtime is a different matter, they pretty much never let you change after the call has been made.