I believe you can just go into the properties for the sheet and change the sheet format to the desired size....
well it is as simple as BOB said. Some thins to look for is that the information on the title block gets transferred to the new title block.
and that you do not scale the annotations if you change size of the drawing views.
You have two paths.
1. Right click on white space and select properties, then replaces the sheet format with the new sheet format. This requires you to go into the 11x17 drawing and save the sheet format somewhere first.
2. Copy and paste the views from the old drawing to the new one .
I recommend the first choice. But both options require that your company has sheet formats set up to accept common drawing properties, which is very likely the case but not necessarily.