Use the following scripts.
Rename by pattern in current directory:
for f in <pattern: *.md>; do mv "$f" "$(echo "$f" | sed s/OLDNAME/NEWNAME/)"; done
Rename by search pattern recursively:
find . -name '<pattern: *.md>' -exec bash -c 'echo mv $0 ${0/OLDNAME/NEWNAME}' {} \;