[afterburner]

Most descriptions of the changes don't go into nearly enough detail. Even pilots have a very abbreviated understanding of everything that's going on, using rules of thumb that gloss over nuance in calculating the stability of an aircraft. Probably a lot of aircraft engineers don't have a full understanding in general unless they're forced to "do the math" on a particular plane.

Making the engine more powerful (as is the case here) means there will likely be more pitching up moment (turning about the aerodynamic centre or AC) due to the position of the engine relative to the AC. Speaking of which, there is an AC, an aerodynamic centre that is separate from the center of gravity. It represents the point about which the plane pitchs up/down when a force is exerted on it.

Not only is the engine more powerful, but the different position it was placed in (forward primarily) would also create a different force about the AC, and change the forces the other parts apply to the plane (by moving the AC and CG).

There might have been more to it but even a few months after really diving into this the details are getting fuzzy... plane stability dynamics can vary greatly.