They are jpg and that is not changed in the renaming but a 1 Meg image is reduced to a few hundred Kbs after the renaming process. I do NOT have File type (Conversion) checked. I just need to rename files because my camera regenerates the same image numbers as file names. What do I need to do to keep the same image quality during the renaming process?
Using the 'process multiple files' in the editor means opening each file, applying the selected options, even if it's renaming only. Then the file is re-saved. If it's a jpeg, that means a new compression. You can get a smaller or bigger file size, never the same.
You can try to change the option for the jpeg size to get about the same size, but the file will be recompressed anyway.
In your case, renaming the file is not necessary if you use the organizer. With my old Canon cameras, I have many different files with the same file names. The organizer recognizes they are not duplicates and will allow importing them. If your destination choice results in importing in the same subfolder, automatically, a suffix will be added at import time.
If you have a set of files you want to rename only, use the organizer to 'export as new files' and choose a common starting name.
If you want the best flexibility in renaming, use the advanced options of the downloader at import time. It's the last option in the advanced modification of the name.