As Luminous said: YS75 "If taking on another ability reduces your character's refresh to zero or otherwise fulfills all the "musts" of a faerie of the appropriate type, the changeling's Chice is made and he becomes a full faerie, never mortal again, and now fully subject to the will of the Faerie Courts."
The underlined portion means "If you now have all the powers of your fae parent, you are now fully fae (an NPC)."
This is your assumption, not something that's explicit on the text. Actually, the fact that the text specifically points out that a Changeling that chooses to become a full faeries "often becoming an NPC as his power costs rise to exceed his refresh" suggests that NPC-dom is a consequence of running out of refresh and not something that always happens to Faerie PCs, otherwise that "often" would have been and "always", and the note about running out of refresh wouldn't be necessary.
Also, for all the talk about monster characters not having full will and going against the novels themes, White Court Vampires are full monsters that only look (but are not) human, and are explicitly suggested as a PC template for high refresh games. Which makes perfect sense, since Thomas would obviously be a PC. Heck, Mouse is suggested as a possible PC, and he's clearly not human.
Eduardo Penna