welcome to the forums.
To change the primary IP to the value of the secondary, you have to:
- first delete the secondary IP address
- then change the primary to the secondary address
I assume that all of your traffic will use the secondary at the moment (default gateway for the LAN, routing etc.). The bad news is that deleting the secondary will stop all traffic from your clients across the FGT.
The good news is that changing the primary address will be effective immediately after pressing OK in the GUI. And as both IP addresses have used the same interface, thus the same MAC address, there won' t be any timeouts caused by a new MAC address.
Same holds true if you use the CLI, doesn' t make any difference.
Second question about SNMP: checking the SNMP checkbox in the interface config will only allow SNMP queries on that interface. SNMP configuration is done elsewhere in the GUI (setting communities, trap target etc.). Depends on your FortiOS version. This is clearly documented in the Admin Guide, though.
"Kernel panic: Aiee, killing interrupt handler!"