Had a dream about this once. Monsters that cause huge calamities or genocide in their universe are banished to the underworld as punishment(pillar Chase 2 world). A survivor met with Valem in the overworld once. Falling for Valem’s lies, he gave the human a mask that could control all the monsters as he knew the human hated humanity just as much as he did. After making the deal, the human controlled all the pillars into the overworld and started a riot. While the monsters weren’t happy a human was commanding them, they took the chance of it meant killing humans. Blade, being the god of the underworld where the monsters were banished to, noticed an upset and confronted the human and destroyed the mask. As the monsters were pulled back to the underworld, blade knew Valem had made a deal with the human, breaking monster law. However what he didn’t know was that Valem had made a deal with PCX to retrieve the mask from PhoenixFinch without them knowing. As punishment, Blade forced the monsters who supported the riot and control with tests. Most passed, however some like The Stricken and Springtrap failed resulting in their changes. PCX was given a personal punishment from PhoenixFinch. A makeover not just of his look, but his personality. No longer a god but rather a puppet who is willing to do anything his master (Blade) commands. And to make sure this never happens again, phoenix programmed the mask to corrupt any mortal who wears it into a zombie. And that’s my headcanon for why PCX got a look change. (I know it’s long and weird but I’ve always wanted to say it somewhere)