How to rename files in bulk from command line?

Question

When I copy files from my camera to my computer the filename extension is upper case. I found this question and answer but the proposed solution doesn't work.

My question is why does the proposed solution not work and how do I accomplish renaming the files such that I am only modifying the filename by making the extension lowercase?

NOTE: The files are stored on a NAS (Synology DS1813+).

Here is my attempt to rename the files:

$ ls
IMG_8809.JPG  IMG_8813.JPG  IMG_8817.JPG  IMG_8821.JPG  IMG_8825.JPG  IMG_8829.JPG  IMG_8833.JPG  IMG_8837.JPG  IMG_8841.JPG  IMG_8845.JPG  IMG_8849.JPG  Thumbs.db
IMG_8810.JPG  IMG_8814.JPG  IMG_8818.JPG  IMG_8822.JPG  IMG_8826.JPG  IMG_8830.JPG  IMG_8834.JPG  IMG_8838.JPG  IMG_8842.JPG  IMG_8846.JPG  IMG_8850.JPG
IMG_8811.JPG  IMG_8815.JPG  IMG_8819.JPG  IMG_8823.JPG  IMG_8827.JPG  IMG_8831.JPG  IMG_8835.JPG  IMG_8839.JPG  IMG_8843.JPG  IMG_8847.JPG  IMG_8851.JPG
IMG_8812.JPG  IMG_8816.JPG  IMG_8820.JPG  IMG_8824.JPG  IMG_8828.JPG  IMG_8832.JPG  IMG_8836.JPG  IMG_8840.JPG  IMG_8844.JPG  IMG_8848.JPG  IMG_8852.JPG
$ find . -name '*.*' -exec sh -c 'a=$(echo {} | sed -r "s/([^.]*)\$/\L\1/"); [ "$a" != "{}" ] && mv "{}" "$a" ' \;
mv: ‘./IMG_8835.JPG’ and ‘./IMG_8835.jpg’ are the same file
mv: ‘./IMG_8822.JPG’ and ‘./IMG_8822.jpg’ are the same file
mv: ‘./IMG_8830.JPG’ and ‘./IMG_8830.jpg’ are the same file
mv: ‘./IMG_8850.JPG’ and ‘./IMG_8850.jpg’ are the same file
mv: ‘./IMG_8820.JPG’ and ‘./IMG_8820.jpg’ are the same file
mv: ‘./IMG_8826.JPG’ and ‘./IMG_8826.jpg’ are the same file
mv: ‘./IMG_8847.JPG’ and ‘./IMG_8847.jpg’ are the same file
mv: ‘./IMG_8816.JPG’ and ‘./IMG_8816.jpg’ are the same file
mv: ‘./IMG_8819.JPG’ and ‘./IMG_8819.jpg’ are the same file
mv: ‘./IMG_8814.JPG’ and ‘./IMG_8814.jpg’ are the same file
mv: ‘./IMG_8810.JPG’ and ‘./IMG_8810.jpg’ are the same file
mv: ‘./IMG_8817.JPG’ and ‘./IMG_8817.jpg’ are the same file
mv: ‘./IMG_8809.JPG’ and ‘./IMG_8809.jpg’ are the same file
mv: ‘./IMG_8851.JPG’ and ‘./IMG_8851.jpg’ are the same file
mv: ‘./IMG_8844.JPG’ and ‘./IMG_8844.jpg’ are the same file
mv: ‘./IMG_8818.JPG’ and ‘./IMG_8818.jpg’ are the same file
mv: ‘./IMG_8823.JPG’ and ‘./IMG_8823.jpg’ are the same file
mv: ‘./IMG_8848.JPG’ and ‘./IMG_8848.jpg’ are the same file
mv: ‘./IMG_8842.JPG’ and ‘./IMG_8842.jpg’ are the same file
mv: ‘./IMG_8815.JPG’ and ‘./IMG_8815.jpg’ are the same file
mv: ‘./IMG_8837.JPG’ and ‘./IMG_8837.jpg’ are the same file
mv: ‘./IMG_8827.JPG’ and ‘./IMG_8827.jpg’ are the same file
mv: ‘./IMG_8825.JPG’ and ‘./IMG_8825.jpg’ are the same file
mv: ‘./IMG_8840.JPG’ and ‘./IMG_8840.jpg’ are the same file
mv: ‘./IMG_8812.JPG’ and ‘./IMG_8812.jpg’ are the same file
mv: ‘./IMG_8811.JPG’ and ‘./IMG_8811.jpg’ are the same file
mv: ‘./IMG_8836.JPG’ and ‘./IMG_8836.jpg’ are the same file
mv: ‘./IMG_8828.JPG’ and ‘./IMG_8828.jpg’ are the same file
mv: ‘./IMG_8832.JPG’ and ‘./IMG_8832.jpg’ are the same file
mv: ‘./IMG_8839.JPG’ and ‘./IMG_8839.jpg’ are the same file
mv: ‘./IMG_8831.JPG’ and ‘./IMG_8831.jpg’ are the same file
mv: ‘./IMG_8824.JPG’ and ‘./IMG_8824.jpg’ are the same file
mv: ‘./IMG_8813.JPG’ and ‘./IMG_8813.jpg’ are the same file
mv: ‘./IMG_8843.JPG’ and ‘./IMG_8843.jpg’ are the same file
mv: ‘./IMG_8846.JPG’ and ‘./IMG_8846.jpg’ are the same file
mv: ‘./IMG_8829.JPG’ and ‘./IMG_8829.jpg’ are the same file
mv: ‘./IMG_8821.JPG’ and ‘./IMG_8821.jpg’ are the same file
mv: ‘./IMG_8838.JPG’ and ‘./IMG_8838.jpg’ are the same file
mv: ‘./IMG_8833.JPG’ and ‘./IMG_8833.jpg’ are the same file
mv: ‘./IMG_8834.JPG’ and ‘./IMG_8834.jpg’ are the same file
mv: ‘./IMG_8845.JPG’ and ‘./IMG_8845.jpg’ are the same file
mv: ‘./IMG_8841.JPG’ and ‘./IMG_8841.jpg’ are the same file
mv: ‘./IMG_8852.JPG’ and ‘./IMG_8852.jpg’ are the same file
mv: ‘./IMG_8849.JPG’ and ‘./IMG_8849.jpg’ are the same file
$ ls
IMG_8809.JPG  IMG_8813.JPG  IMG_8817.JPG  IMG_8821.JPG  IMG_8825.JPG  IMG_8829.JPG  IMG_8833.JPG  IMG_8837.JPG  IMG_8841.JPG  IMG_8845.JPG  IMG_8849.JPG  Thumbs.db
IMG_8810.JPG  IMG_8814.JPG  IMG_8818.JPG  IMG_8822.JPG  IMG_8826.JPG  IMG_8830.JPG  IMG_8834.JPG  IMG_8838.JPG  IMG_8842.JPG  IMG_8846.JPG  IMG_8850.JPG
IMG_8811.JPG  IMG_8815.JPG  IMG_8819.JPG  IMG_8823.JPG  IMG_8827.JPG  IMG_8831.JPG  IMG_8835.JPG  IMG_8839.JPG  IMG_8843.JPG  IMG_8847.JPG  IMG_8851.JPG
IMG_8812.JPG  IMG_8816.JPG  IMG_8820.JPG  IMG_8824.JPG  IMG_8828.JPG  IMG_8832.JPG  IMG_8836.JPG  IMG_8840.JPG  IMG_8844.JPG  IMG_8848.JPG  IMG_8852.JPG

I also tried this command with the same results:

$ rename s/.JPG/.jpg/ *.JPG
IMG_8809.JPG not renamed: IMG_8809.jpg already exists
IMG_8810.JPG not renamed: IMG_8810.jpg already exists
IMG_8811.JPG not renamed: IMG_8811.jpg already exists
IMG_8812.JPG not renamed: IMG_8812.jpg already exists
IMG_8813.JPG not renamed: IMG_8813.jpg already exists
IMG_8814.JPG not renamed: IMG_8814.jpg already exists
IMG_8815.JPG not renamed: IMG_8815.jpg already exists
IMG_8816.JPG not renamed: IMG_8816.jpg already exists
IMG_8817.JPG not renamed: IMG_8817.jpg already exists
IMG_8818.JPG not renamed: IMG_8818.jpg already exists
IMG_8819.JPG not renamed: IMG_8819.jpg already exists
IMG_8820.JPG not renamed: IMG_8820.jpg already exists
IMG_8821.JPG not renamed: IMG_8821.jpg already exists
IMG_8822.JPG not renamed: IMG_8822.jpg already exists
IMG_8823.JPG not renamed: IMG_8823.jpg already exists
IMG_8824.JPG not renamed: IMG_8824.jpg already exists
IMG_8825.JPG not renamed: IMG_8825.jpg already exists
IMG_8826.JPG not renamed: IMG_8826.jpg already exists
IMG_8827.JPG not renamed: IMG_8827.jpg already exists
IMG_8828.JPG not renamed: IMG_8828.jpg already exists
IMG_8829.JPG not renamed: IMG_8829.jpg already exists
IMG_8830.JPG not renamed: IMG_8830.jpg already exists
IMG_8831.JPG not renamed: IMG_8831.jpg already exists
IMG_8832.JPG not renamed: IMG_8832.jpg already exists
IMG_8833.JPG not renamed: IMG_8833.jpg already exists
IMG_8834.JPG not renamed: IMG_8834.jpg already exists
IMG_8835.JPG not renamed: IMG_8835.jpg already exists
IMG_8836.JPG not renamed: IMG_8836.jpg already exists
IMG_8837.JPG not renamed: IMG_8837.jpg already exists
IMG_8838.JPG not renamed: IMG_8838.jpg already exists
IMG_8839.JPG not renamed: IMG_8839.jpg already exists
IMG_8840.JPG not renamed: IMG_8840.jpg already exists
IMG_8841.JPG not renamed: IMG_8841.jpg already exists
IMG_8842.JPG not renamed: IMG_8842.jpg already exists
IMG_8843.JPG not renamed: IMG_8843.jpg already exists
IMG_8844.JPG not renamed: IMG_8844.jpg already exists
IMG_8845.JPG not renamed: IMG_8845.jpg already exists
IMG_8846.JPG not renamed: IMG_8846.jpg already exists
IMG_8847.JPG not renamed: IMG_8847.jpg already exists
IMG_8848.JPG not renamed: IMG_8848.jpg already exists
IMG_8849.JPG not renamed: IMG_8849.jpg already exists
IMG_8850.JPG not renamed: IMG_8850.jpg already exists
IMG_8851.JPG not renamed: IMG_8851.jpg already exists
IMG_8852.JPG not renamed: IMG_8852.jpg already exists

---

EDIT

See this question about file systems and case sensitivity issues.

Answer 1

A friend pointed me to this. The linked article is 8 pages long, but it is verbose in its explanation. The short version of the answer is:

... And that brings me to what I really want to discuss, which is how 
CIFS Server (Samba) deals with this.

As you would expect from an application that was grown to bridge 
the gap between the Unix and Windows worlds, it is very flexible. 
This is both good and bad - with flexibility comes responsibility, 
and sometimes not a little confusion. There are four configuration 
options that Samba provides to allow one to define its behavior 
when dealing with matters of 'case': 

preserve case = (yes/no) 
short preserve case = (yes/no) 
default case = (upper/lower) 
case sensitive = (yes/no)

The solution is to configure Samba to handle case sensitivty (or case preservation issues) the way that works for your specific needs.