IANA defined "well-known port" for Diameter is 3868; recent RFC 6733 defines 5658 for TLS/DTLS secured Diameter connections, leaving 3868 for plain TCP/SCTP based connections.
Implementations do typically allow you to use a different listening port; when a node uses a different port, its peers should be appropriately configured.
I don't know much about DPI. But since DPI would have the ability to recognize patterns within the PDUs as well as in protocol exchanges, shouldn't be too difficult. Anyway it would need the command and AVP definitions to interpret the traffic even when server is listening on 3868. Here, it has to go a step beyond - 'guessing' that protocol is Diameter, validating the guess and then applying its knowledge of the PDU structure.