You can use a different IP address to answer for the SSL VPN.
Lets say that your interface IP (The default IP address that is used with the SSL VPN) already has HTTPS (443) forwarded in to a internal server, and you really want the SSL VPN port to be 443. You have an option.
You can add a secondary IP address under the WAN interface that does not have a reservation already for 443. Then use this IP address for the SSL VPN.
To do so:
Add your secondary IP address – Note this has to be a public address, given to you by your ISP..
Then go into the VPN settings and modify the port for what you want. Notice that the address it says will work is still the primary IP, even though the secondary will work just fine.