Like most services,
afpd is controlled by
launched which responds to incoming network requests.
I haven't tried it, but it
should be as simple as editing /System/Library/LaunchDaemons/com.apple.AppleFileServer.plist and editing the <Sockets> section to list the port you want:
<key>Listener</key>
<dict>
<key>Bonjour</key>
<true/>
<key>SockServiceName</key>
<string>
afpovertcp</string>
</dict>
afpovertcp is the service name associated with AFP (namely port 548). Replace this with a different port number (or service name) as appropriate. You'll need to restart the service in order for the change to take effect.
Note that there are other caveats - I don't know, for example, whether other services that rely on AFP will continue to work - for example, it's possible that Server Admin may not be able to monitor the service, but I haven't tried this to check.