That's all good and dandy. You do know this thread is lightyears old
So are you mainly concern with cpsg gateways at the remote and vpns? If the end-devices are CPSG and your want to migrate off the central HQ 3000 to let's say a FGTXXXXX, you could build a new vpn-community, apply the gateway address of the FGT and then install that policy to redirect that "spoke" to the new HUB.
And then disable the old policy at the CHKP 3000 and adjust for any routing thru the new fortigate. I worked a project that was just like the above with walking over vpn-spokes one at a time and it was doable. Afterward we monitor the rule and encryption/decryption details in the eventlogs to ensure that new tunnel was up or use vpn tunnelutility. After you figure out the plan and steps, you could easily migrate a few per night or during a maintenance window.
Just my 2cts, and god I hate CHKP