What I suggested was to change only the main WAN address to A.B.C.2, which will enable you to use Remote Access VPN on that address.
You can keep the published pages and services (except VPN) on IP address A.B.C.1 or any other address of your range, provided you set the correct proxy ARP and NAT rules.
This seems to be the way to cause least impact on your partners and VPN clients.
The only affected services would be site-to-site VPNs, if you have any, which will have to move from A.B.C.1 to A.B.C.2, but I think it is better to make changes to site-to-site than client-to-site, specially if you don't have a DNS for that.
No impact for published pages
No impact for VPN clients
Easy to fix impact on site-to-site VPNs.