There's no real difference, it's just what you can configure in the GUI for the interfaces. I think it's mainly used for the self auditing feature in 5.6. DMZ interfaces have different rules that apply to the checks than LAN ones.
Usually some objects/settings break if you do not follow the upgrade path and/or on top of this some of the conversion scripts used in the upgrade process will convert certain settings that may no longer be supported on new firmware or converts some settings to defaults.
If your foritgate stores config revisions, I suggest comparing the current config vs the old config and see if what's changed or use a text compare tool.
That said, performs perform a exec dhcp lease-list from the CLI and see if anything shows up. If need be, perhaps purge the DHCP leases (e.g. exec dhcp lease-clear all ).
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