I've had this interaction twice in slightly different situations.

The first time I had a Silent Hallcreeper imprinted with Hidden Strings and a Thing in the Ice on the board. I attacked with Silent Hallcreeper, targeted with with Secret Identity, then dealt combat damage. I chose the draw a card option, then Hidden Strings was triggered. I can't recall what I targeted with Hidden Strings but it removed the last ice counter from Thing in the Ice, causing it to transform. When it did, it sent Silent Hallcreeper back to my hand.

The second time, I had the same setup on the board with hallcreeper and thing but also had a Bloodline Pretender (I chose horror). I targeted Bloodline Pretender with Secret Identity, dealt combat damage with Silent Hallcreeper, triggered Hidden Strings, and flipped Thing in the Ice. Bloodline Pretender was returned to my hand.

I'm not exactly sure why but I assume Secret Identity is making its target a hero and only a hero but the card does not state it loses other creature types. Even if it did, it doesn't remove changeling from Bloodline Pretender.

Thanks for all you do! This might get lost in the noise of complainers but you people are awesome.