I would go to Immigration and cancel the old visa just to be safe.. The way immigration is here you want to have it as clean as possible. From my experience at least....
I'm about to do my 12 month extension for the first time. Do you know if the bank letter/ statement needs to be done on the same day as the appointment or is the day before fine? Thanks for any information you can provide (I'm a little worried about having the correct paperwork)