It bothered me that changelings have no mechanics to reflect their male parentage. Surely a changeling lass sired by an orc is going to be much different than one sired by a halfling!
So I came up with this feat based on Racial Heritage and gave it to all changelings in my campaign:
You were born of the union of a hag and her victim.
Benefit: Choose your humanoid father's race. You count as both a changeling and that race for any effects related to race. For example, if you choose human, you are considered both a changeling and a human for the purpose of taking traits, feats, how spells and magic items affect you, and so on.
Mechanically, it lets changelings qualify for things like the Razortusk feat (for a half-orc changeling), or the Fast Learner feat (for a human changeling).
In terms or lore, it would provide useful camouflage for the young changeling. When someone tries to teach her something, she can learn it in the normal way, and thus not creep people out any more than the two-tone eyes already do. Ditto for reacting to magic in race-appropriate ways.
Naturally any changeling who accepts her mother's call and transforms into a full hag would lose this.
I'm not good judging how things are in game balance terms but flavor-wise I like it a lot.
You say you gave it to all changelings but, being a feat would it be optional? Do they get it as a free feat?
My changeling has elven heritage and I picked the adopted trait to reflect it, it works OK but I like your option.