“Entry in use” Error When Changing WAN1 Addressing Mode

Hi, we have a single Fortigate 80E running 6.0.10. We are switching internet service providers going from cable to Fibe. We need to change the WAN1 addressing mode from MANUAL to PPPoE. When attempting to make the change on the WAN1 interface and save via the GUI, we get an “Entry is used” error message and no changes are made. We have a few VPN tunnels and approx 30 policies that reference WAN1. Do we have to change or remove all current references to WAN1 before we can change from Manual IP to PPPoE ? Appreciate any guidance on this..... Thanks