This. Especially with award tickets. I recently had a very similar situation as you on a J award to Europe. The first agent I called was unwilling to help me because the schedule change "wasn't substantial" (it was ~1hr), the second was unwilling to help because there weren't award seats available on the flights I wanted, and the third did exactly what I asked (extended my return by a day, overriding award capacity controls) with little hassle. Agents make things up all the time, and the "trip must be the same length rule" you were quoted sounds like just such an invention. Ask nicely; if you meet resistance, just back out of the call and try again with someone else. What you're attempting can definitely be done. Good luck!