I can think of daemons that accept a kill USR1 or NOHUP signal to re-read their configuration... at the software's earliest convenient time. Others like Plackup have an -R to constantly monitor file changes, and reread the config file.

The rest has been said by a fellow monk. The thing is: faulty config files also can make the program crash. So if pushed automatically, and re-read automatically... maybe an automatic rollback and human-signaling should be implemented.