The problem with using the large format driver isn't just the fact that the larger driver doesn't have quite the HF extension. Even if that weren't the case, the horns used with them are necessarily different sizes too. The geometry is wrong for the existing crossover, so you would have to redesign. I've gone to great lengths to set the crossover points right so directivity is uniform at the crossover points and the null angles are outside the pattern. That's not a trivial task.