Complex sentence: A very early change in design was changing the horizontal tail group from straight to a 15-degree dihedral to improve stability, while another significant modification was altering the nacelles to have exhaust stacks, retroactively incorporated after the series entered production.
Split into simpler sentences: A very early change in design was altering the horizontal tail group to add a 15-degree dihedral to improve stability. Another significant modification was altering the engine nacelles to have exhaust stacks, a modification that was retroactively incorporated after the series entered production.