D&D 5E Changelings and Scrying

I'm having issues with trying to find how this would interact with a shape-shifted changeling. The changeling in question has kept a very tight lid on the fact that he's not actually human. To his knowledge, the only people who know are a few in his backstory. Before recent events, there wasn't any reason for the particular people who would be looking for him now to know anything about his past, as he changes his day-to-day persona every several years.

With the wording of the scrying spell, you must know of the target of the spell in some capacity.

While the current persona is known by the scrying parties, if he were to shift into another form while they weren't watching, how would the scrying spell interact? Would it fail due to not having a viable target, or does the scrying spell transcend past the innate ability of the changeling?

I would consider giving a penalty to the scrying check if he has changed his persona, but still give a bunus if they have a personal item that belonged to him. So, it would be harder, but not impossible.

Scry without any accoutrements just works on a person, so if someone just casts scry and picks him, then they apply the 'secondhand' or 'firsthand' modifier, and they get him, no matter what he looks like.

HOWEVER - he probably never counts as "known well" if he's living a double life. The "likeness or picture" bonus probably does not apply if he's not in the correct likeness (and it may even count as a penalty). I would think that 'body part' would always apply however, regardless of polymorphed status.

If he wants to maintain his disguise, then he probably wants to use a nondetection spell.