The 20D filenames, folders names, and organization are fixed in the software built into the camera. Short of a hack of the firmware, you are not going to be able to change the filenames. But the chance of overwriting existing files is very low. First they would have to have the cr2 extension and second they would have to have the exact same file number (a number from 0001 to 9999). If you are shooting with more than one 20D or Canon camera that uses the CR2 extension and you insist in keeping the files in the same folder, then you should get a product called Downloader Pro from BreezeSys which will automatically copy the files from your CF cards to the folder of your choice and rename the files in anyway you want. It also can be setup to take care of file name duplication should you try to download the same photos twice. Here's a fast link for more information.