It could happen after plebe year.
Or youngster (sophomore) year.
Or not at all.
Pros and cons for all approaches.
It’s actually good training for the Fleet and Corps. Rule of thumb is 30% of a unit changes every year, so new leadership, new shipmates, new sailors/Marines.
It’s also good training for when you are doing well on your ship and your quals, just a few months back from deployment, when the officer in your same job, same class of ship across the pier, broke her leg and now can’t deploy next week, so you get loaned to that ship for a period of time, and off you go. Of course, due to your early training at USNA that the one constant of military life is change, you roll with it and excel, quickly learning the people, culture, etc.