Yesterday I migrated one of my services from one server to another. Since the protocol used by the service does not support a HTTP-esque redirect and the Windows Server version used did not have the RRAS roles available, I had to get a little creative.
Enter Komodia Relay, a great (and free, to boot!) tool to forward a TCP/UDP port to a different system. The basic idea here is that it works like a proxy, clients connecting to the old server will transparently be proxied to the new one through Komodia Relay.
Usage is outstandingly easy and even under loads of several hundred connections the application still performs beautifully.
If you are more the GUI-oriented type and do not mind to pay for your ride, Network Activ’s AUTAPF might be worth checking out.