It depends on what you are switching from and to. If you are staying within the department it is much easier.
Engine Systems to Engine or Engine to S/Y Management ... Fairly Easy
Deck to Engine (vice versa) ... not so much.
I saw people change majors as late as second class year within the department, but typically it is no later than third class year when switching departments.
This was of course on the quarter system. I don't know different it is on the trimester system.