Help clarification with unchained eidolon

[Adamarh]

Hi, i want to build a summoner and i start planning what evolution i want but for the Large evolution its written
"The ability increase evolution costs twice as much (4 evolution points) when adding to the Strength or Constitution scores of a Large or Huge eidolon. Requirements: Summoner level 8th (13th for Huge), Medium size."

Is it retroactive ? So i must pay 6 point to become Large if i already have one increase in STR ?( 4 for the large evo + 2 for the difference between what i paid the first time 2 and the cost after it become large 4)
Or does it only increase the cost after taking the feat? If it does, does that mean that using Transmogrify can make me lose evolution point cause it reallocate the count of point?

[Melkiador]

You reassign evolution points every time you level up. So, retroactive is mostly irrelevant, other than evolution surge.

Quote:

each eidolon receives a pool of evolution points, based on the summoner’s class level, that can be used to give the eidolon different abilities and powers. Whenever the summoner gains a level, he must decide how these points are spent, and they are set until he gains another level of summoner.

When you are leveling, you choose large and then decide if you want to keep the more expensive ability increase evolution or not. Transmogrify is basically the same deal. You start over with a new pool and choose which evolutions you want. Your previous set of evolutions is irrelevant.

I'm not certain what would happen with Evolution Surge. My guess is that you keep the ability increases while you have a surged large evolution, otherwise it gets wonky with having to decide which evolutions to temporarily cut.