There should be a reauthentication request that occurs on registration to change the role.
This could be done by RFC 3576/5176 CoA/DM, toggle link, or possibly an SNMP set. It depends on which Control solution is in play and what the switch supports.
With ZTP+ I'm assuming you have either EXOS/VSP switches that you're working with. It would then depend on if the reauthentication method that is being used is causing a toggle link or link bounce in order to get the DHCP services to initialize on the end system.
With EXOS I don't believe a link bounce is supported (yet).
What type of switch and which control solution are you working with?