I have done a template with drawing frame in a A3 format, then I saved a copy to create a A1 drawing template but the printing arena is still a A3. I can see in the lower position of the screen it says SIZE: A3. When I double click at it pop up with a box where I can choose Define and after that I can select : Grid Outline, Grid Origin, Columns, Rows and Done/Return. Am I out on a wrong road?
what extension does have the file containing your "template" ... is it .drw or .frm ? Can you upload the file (How to upload Creo (or other) files) ?
Drawings (and templates) can have different size formats applied to them, but I do not believe you can change the size of a format itself after it has been created.
I made a product idea for this since I run into it every time I need to make a new format: