In FortiOS 7.6, when SD-WAN is enabled, physical and logical WAN interfaces are added as SD-WAN members and are abstracted behind the SD-WAN interface (virtual-wan-link or SD-WAN zone) . Traffic forwarding decisions are then made by SD-WAN rules instead of individual interfaces.

As documented in the FCSS SD-WAN 7.6 curriculum and Fortinet SD-WAN architecture guides, firewall policies must reference the SD-WAN interface or SD-WAN zone , not the individual WAN interfaces that are members of SD-WAN. Therefore, during the configuration update process, existing firewall policies that reference physical WAN interfaces must be updated to reference the SD-WAN interface .

Option A is incorrect because routing configuration does not require replacing interface references when SD-WAN is enabled. Static and dynamic routes typically point to the SD-WAN interface automatically, and SD-WAN rules handle path selection.

Option B is incorrect because SD-WAN is a built-in FortiOS feature . It does not require a separate license and does not require a reboot when enabled.

Option D is incorrect because interfaces must remain enabled to function as SD-WAN members. Disabling an interface would prevent SD-WAN from using it for traffic forwarding.

Therefore, the required action during the SD-WAN configuration update process is to replace references to interfaces used as SD-WAN members in the firewall policies , which corresponds to option C.