Instead of enhancement cores, just leave old gear behind by having a lower power level cap for some period of time. E.g. All gear prior to forsaken caps at 625 and forsaken gear caps at 700. Then after some period of time (two to three months), open the flood gates to older gear. This would allow for people to experience the new challenge as designed, naturally incentivize new gear without having to nerf prior things, create meta shifts for light based competitive activities and eventually things would open up for more options and a bit more chaotic fun that would ease the entry into more the recent difficult activities (raids and what not).