User blog comment:MetallicArcher/Tayang Xiao Long and Penny's last Name confirmed?/@comment-24245869-20140910034750/@comment-24435688-20140910040736

It was said at some point in the last RWBY Livestream. Monty said that Neptune was originally on a different team, but they decided that they would switch him with the previous N of SSSN and use him now.

I can't give you an exact time, but you might as well watch it all if you haven't already.