Odd.
1) I had not thought of pressing enter.
2) It does/did not work for me. All I now see is it change back to 5190 (rather than changing to Account Info pane and back)
3) Further testing of deleting the port 5190 one character at a time and playing with the SSL option a couple of time got it to stay on a second account.
In fact it was an @Mac.com name which waited for a while then reported the SSL would not work with the Apple Server.
To restate:-
I had SSL Off (It seemed to be the default).
I had to Enable SSL to get the Change to accept when pressing enter. (I could turn Off the SSL again and the change stayed before Logging back in again.
7:44 PM Saturday; September 5, 2009