The files "/etc/sysconfig/[dhcpd|dhcpd6|dhcrelay]" used for specifying user's 
customization arguments are deprecated in CentOS7. Instead, these arguments can
be specified in "[dhcpd|dhcpd6|dhcrelay].service" files on "ExecStart" line.

The specified arguments are automatically moved from /etc/sysconfig/*
files into *.service files during upgrade. Please check if they are moved
correctly and everything works as expected.