Kevin
If you have a very large number of files that need to be renamed, it can obviously be very tedious, if they can be named in some sort of series or logical pattern however (eg: file1.dwg, file2.dwg, file3.dwg....) then you could use a mass file renaming utility, search
http://www.download.com for something like this.
HTH,
Dan
Edit: possibly something like this:
FileName Magic Pro , I havent personally used it, but it might be worth a shot.