Hi. I configured a XG firewall with a WAN regular dhcp interface.
In the client, I'll have to configure a VLAN ID on the interface. I don't know for sure what it is, so I will have to test more than one.
What is the easiest way to do it ? I saw this to create it from scratch:
https://community.sophos.com/kb/en-us/123127
But I just want to change the current WAN interface specs, so I don't have to change anything else. 
You are not changing it, you are adding a VLAN to the port.
Adding a Vlan to any port
Go To Network
go to Add Interface
Select Add VLAN
Select Physical PORT (by Port ID number select the one that you are using as your WAN port)
Select Zone (WAN in your case)
Select VLAN
Enable IP Configuration and set as desired
You are not changing it, you are adding a VLAN to the port.
Adding a Vlan to any port
Go To Network
go to Add Interface
Select Add VLAN
Select Physical PORT (by Port ID number select the one that you are using as your WAN port)
Select Zone (WAN in your case)
Select VLAN
Enable IP Configuration and set as desired
