Interesting question. I have never had the opportunity to change the production environment from static to dynamic.
All of the below is a speculation and you should NOT rely solely on this information.
Please note that changing your gateway's WAN IP and selecting "Dynamic Address" in gateway's properties will remove most of the blades from the objects, will wipe the NAT rules and objects' properties and will most definitely demolish whatever VPNs you have configured.
Please check the properties of the gateway object.
In the main window, check if the IP address assigned to the gateway [1] is the one you are trying to change to DAIP.
If so, there is option to make it "dynamic" [2]
Before doing that, you have to change the parameters of the corresponding physical interface in Gaia WebUI from static IP to "Obtain IPv4 address automatically"
In the "Network Management" section, execute "Get Interfaces" without topology.
I suspect that at this point, the WAN interface is associated with some-kind of dynamic object.
If the gateway's IP address in the "General Properties" is NOT your WAN interface's IP, I would not know how to approach this change.
If someone has the experience with this, please chime in.
Regards,
Vladimir