Well with the Army the truth is at least with communication and medical (because that is what I know about first hand) when you go to your AIT if you don't like your MOS alot of people will just fail out on purpose and re-class.
And another situation I know about first hand that is probably more relevant to your question. My friend back at home joined the National Guard active first program and after he swore in he decided that the MOS wasnt for him, and (BEFORE) he left for basic training he changed his MOS the only problem for him was doing that pushed his ship date for basic back several months.