We recently came across this strange issue when switching static IP providers on a UDM Pro. In the new UI the change would simply not take effect and error out. In the classic UI an "outband NAT" error prevented the change. This is surely a UDM Pro bug, as the UDM Pro won't let you change the primary WAN as the IP address of the existing service is tied to the LAN networks. 

The fix is below. 

1. Add an additional IP dummy address to the WAN interface. (for instance adding 9.9.9.9/24)
2. Deactivate the New User Interface under System Settings.
3. Change the address in each network in Network / Network Name (i.e. LAN) / Advanced tab / "Internet Source IP" to the new address that you created.
   This must be done for every network that you have set up.
4. Go to WAN settings and enter in the new Primary WAN IP Address.
5. Change the "Internet Source IP" of all your networks to the new primary IP.
6. Remove the additional IP from your WAN settings.

Credit from this Reddit post. It works and you don't have to go crazy and factory reset your UDM.