Suppose that a medium sized Eidolon has spent 2 evolution points for the evolution of Skill Increase (Strength). Later, the Summoner gains a level and gives the Eidolon the great evolution.
According to the great evolution,
The ability to increase evolution costs double (4 evolution points) when added to the Strength or Constitution scores of a large or huge eidolon.
In addition, by reading RAW from the Eidolon class characteristic of Summoner, its evolution points are reassigned to each level.
Each time the invoker wins a level, he must decide how these points are spent, and they are established until he gains another invoker level.
When Eidolon wins the Big evolution, does this retroactively increase the cost of the evolution of the Skill Increase already purchased (from 2 points to 4 points)? Or does this only increase the cost of future purchases of the evolution of Capacity increase, such as through the surge of evolution spell out?