The reason the rumor of shailene being recast is she's going to be in her version of "the hunger games" called "Divergent"
She's getting cut from ASM 2 because it might be too crowded or just doesn't fit the story
If she can't squeeze in ASM 3 then she'll be recast
We might never even see her scenes and a new MJ will be in ASM 3 and no one will know about Shailenes MJ
If Divergent becomes a big hit, she might not have time to do ASM3 too.
Not sure why she can't find the time for both like what Jennifer Lawrence is doing with X-men and The Hunger Games.