If you want to rename the extension of several files into another extension , then this script may help. This is an example of how-to for JPG to jpg conversion that I found on the web :
for i in *.JPG;
do mv $i `basename $i JPG`jpg;
done
replace JPG and jpg for your convenience to any extension you want to use.
Usage :
Move all the file you want to operate on to a folder and place this script in a bash file and place it inside the same folder , and then go to that folder and run the bash script file in terminal.