Okay, so I set up a test gallery @
http://testgallery.samskara.org with the current stable version of Coppermine @ 1.5.20 and here is what I found.
I uploaded the files of a film and created two albums, Test 01 and Test 02.
In the album Test 01 I uploaded the original files of poor quality screencaps from the film "The Chronicles of Riddick" and then used the Batch Add Files everything went well. I then wanted to replace them in a batch add format with higher quality screencaps from the same film and uploaded them with the same file names as the first @ THE CHRONICLES OF RIDDICK 0001. When I went to use the Batch Add feature, none of the boxes beside the new files were automatically checked. I went through and checked all the boxes beside the new files and then hit Continue, and they 'uploaded'. I then went to refresh the page and nothing happened. I then went to the Admin Tools and hit the pip to update everything, thumbs, full size and intermediate pictures. Nothing.
In the album Test 02 I uploaded the original files of lesser quality to this album. I used the Batch Add Files option and everything uploaded fine. I then had to rename the higher quality screencaps thusly: THE_CHRONICLES_OF_RIDDICK_0001 sequentially through to 0314. This was to make sure the file names corresponded to the files already in the album. The
UNDERSCORE is important to fill the empty space between the words. Via FTP Filezilla (you can use whatever FTP program you prefer) I copied the files into the Test 02 album but had to make sure to click on the box that asks if you wish to replace the files in that folder. If not, nothing will happen. I then went to the Admin Tools and updated everything, thumbs, intermediate pictures and large pictures. The replacement worked. All files in Test 02 were of the higher quality images.
I hope that description was helpful. Any questions and I can try to explain it better.
NWN