Always make a backup of the original file before trying this, I find this kind of thing when updating IPs etc.. to use a script.

Where the old IP is "192.168.5.8" and the new IP is "10.10.5.8"

sed -i s/192.168.5.8/10.10.5.8/g testdomain.org.db

The -i with sed means "inline" meaning we edit the file directly, but without the -i we could just use > and output the results to another file or do whatever else we wanted.

Of course instead of it being a zone file, it could be an Apache VHOST just as easily.