If they're in the same user directory, like /home/terrywho/example.com and /home/terrywho/dev.example.com, you can use FTP and just rename those directories:
example.com to example.old
dev.example.com to example.com
example.old to dev.example.com
This will effectively have your domains trade places. Just make sure any internal links and configuration files are updated with the new domain name.