Something like this should get you started:
ls *.htm | while read X
do
NEWNAME=${X}l
if [[ ! -f ${NEWNAME} ]]
then
mv ${X} ${NEWNAME}
else
echo "${NEWNAME} already exists." >&2
fi
done
Because you have some many *.htm files in one directory, you may need to change the ls *.htm to something like ls [a-g]*.htm and do only a portion of themn at a time.
If it ain't broke, I can fix that.