In a cricket match can the captain of the fielding team change the bowler during the over, for any reason aside from necessity (like injury)?
No. At the time of the question, this was covered by Law 22.7(a):
Other than at the end of an innings, a bowler shall finish an over in progress unless he is incapacitated or is suspended under any of the Laws.
With the 2017 rewrite of the Laws, this is covered by Law 17.7.1; the wording is essentially identical.