I never did see FLL like the "use parts from last year" FRC. Plus, unlike FRC, you get about a dozen challenges, and obstacles that may restrict robot height and width. For example, my team 3 years ago built a tall robot, since our mechanisms took most of the bottom area, and there was nothing restricting our height (except the height restriction). The next year, there was a bridge section we decided to go under, but in order to do that, we had to have our robot under about 4 inches (I forget the exact number).

So, I'd say a new drivetrain each year is not only best, but also fun!