I would like the ability to upload and replace an image file that already exists in an album with one with a larger filesize keeping the original filename (ie img_000001.jpg). This would also keep the comments the same.
Upload the file using your FTP app (overwriting the original image), then make sure that coppermine "knows" the new dimensions either by manually editing the database (using phpMyAdmin or similar) or by running the admin tools (option "Reload file dimensions and size information").