Let's see. If every incumbent receives a favorable district, that favoring incumbents. But, if some incumbent receives a less favorable district, that's disfavoring a particular incumbent! And, what is "[dis]favoring an incumbent?" In 99% of the cases, it is [dis]favoring his political party. In the vast majority of cases adding more Republicans helps a Republican incumbent, while a Democrat is best favored by adding Democrats to his district.
New districts would have to be swing districts, else the new district "favors" some party. That is unless new minority districts can be drawn, then, that district can "favor" the candidate of choice of that minority.
But, if we consider "plans" rather than individual districts, we have meta-level claims about overall "partisan fairness," whatever that means. That seems to mean electing more Democrats, which requires redrawing the districts of some Republicans so they are more likely to lose, which is, itself, another violation!
The amendment wasn't designed to reform redistricting. It was designed to spawn litigation with the purpose of moving redistricting to the Courts.
The only way it seems possible to meet the standard is to draw districts so that every Republican incumbent has a slightly less Republican district, [presumably excluding West and Riveria whom would be "disfavored" by any shift at all towards the Democrats] and every Democrat incumbent a slightly less Democratic district, but, not so much that they are seriously jeopardized. Of course, this would have to be done without even calculating the partisan effects of the maps!