Trying to rename them in Windows was painful as I needed to rename it to a temporary extension, commit, then rename back and commit again.
I found a handy script to use in the command line of Linux, and since it supported case-sensitive filenames, all I had to do was rename and commit.
for file in *.PNG ; do svn mv $file `echo $file | sed 's/\(.*\.\)PNG/\1png/'` ; doneWorked like a charm!