There’s pretty much two things that you will need to do.
1. Setup NAT on port 25 from the outside world to the SBS server (if this isn’t already done).
2. Change the MX record on the public DNS server from pointing to the public name for the POP3 server to the public name for the SBS server.
2a. If the SBS server doesn’t have a public name setup yet, then you will need to setup an A record pointing to the SBS servers public IP.
If you have a single IP address that you are doing all this with, then no DNS change is needed. Simply change the router/firewall that is forwarding the port 25 TCP communication from the POP3 server to the SBS server.