This what is normaly used on every device as long it can be configured in interface mode is following:
config firewall policy
NOTE with purge everything within the policy container will be deleted meaning every policy will be deleted. Answer with yes!
config firewall dhcp server
NOTE with purge everything within the dhcp server container will be deleted meaning every entry will be deleted. Answer with yes!
# config sys global
# set internal-switch-mode interface
changing switch mode will reboot the system!
Do you want to continue? (y/n)y
PLEASE NOTE For the newest release of FortiGate 60D you will recognize that the interface mode is ALREADY in interface mode. If you look deeper into it you will recognized that latest revision of FortiGate 60D (first time recognized in January 2015) has a Hardware Switch like the FG-100D up to Revision/Generation 3. If you have such a device you have to splitt out the interface's over the gui. If you do so you will note that the last two interfaces can not be splitted out. The reason is behind this is configured a virtuelle hardware switch which helds the interfaces together. But also this one can be deleted which means:
# config system virtual-switch
== [ lan ]
# del lan
Thats it...have fun