Replacing BM on NW with the ISP firewall and NAT


LAN is a tree with 3 servers:
1. NW 6.5 sp8 + BorderManager 3.9 sp 2
2. NOWS SBE 2.5 (Suse) - DNS\DHCP
3. NOWS SBE 2.0 (Suse)

Since I'm connected to the internet through my ISP router (XBOX- Checkpoint), I am considering to remove the first server (firewall) and ask my ISP ro configure the router as a firewall and NAT too.

What are the steps needed to do it without any demages?