Hi @Benjamin_Olson ,
Instead of stopping the services for the old firewalls I would suggest shutting down the interfaces. IMHO it is
simpler - you can prepare the clish commands for "interface state off" and just copy paste it
faster - faster solution in case of rollback
Couple of months ago I have done something similar for one of our customer - the migration was both software and hardware (switched to new devices running newer version). Our plan was:
1. Prepared the new devices (network config and policy)
2, Shut down the interfaces on the switch and on the firewalls
3. Configured VIP on the few cluster and pushed policy
4. In the day of migration copy pasted the commands to shutdown FW and switch interfaces for the old cluster
5. Copy pasted the commands for enabling interfaces on FW and switch for the new cluster
6. We had issues with one VPN tunnel so we had to rollback and just copy-pasted the interface commands again (down for new and up for old cluster of course)
You can choose to shutdown interfaces only on one device (switch or firewall)