I had also asked this CHANGING PORT question one year ago when i start to use SME server. Since i was not a good Linux user, so i was not able to change port number even i can find the answer here.
Now, in order to fix the problem of blocking port by ISP, i giving up the idea of change the port number of the SME server, rather i change the port mapping on the router side.
I use a SOHO boardband router that allow me to map different internal and external port. That mean for internel (server) port number, i can use 80, while for external (router side) port number, i use other number. As a result, i do not need to do anything on my server.