Calling Season Finale – Changeling Plan Revealed?! · 4:24am Mar 29th, 2016
They are going to replace Flurry Heart. That's why we see the changeling in the episode that introduces Flurry Heart, as well as the one where she's born. Remember Starlight also appeared in episodes relevant to her, the first where there's someone who abandoned their old friend because they were a higher achiever, and the other where they mention the consequences of time travel. The Changeling appears right after the scene with naming Flurry Heart.
In Celtic mythology, changelings replaced babies specifically, to steal love from the family.
The Crystal Empire has always been filled with love and light, but now, there's something that could easily be replaced without notice, that is also the thing with the most love directed towards it besides the Crystal Heart itself. The baby has no personality, that means no changeling could be out of character. Not to mention, the inconceivable amount of power they gain could easily be passed off as baby Alicorn magic spurts if they ever accidentally show it off.