I've edited a movie which I crerated as fullscreen but realise I need to have it widescreen. Is there any way to change this or do I have to redo everything again?
Try copying your 4x3 sequence into a 16x9 sequence which is using the correct preset.
You may need to tell Premiere to interpret footage as widescreen.
A good way to make sure that you have the correct sequence preset is to drag a clip onto the "Make New Item" icon at the bottom of the Project panel