Lucifer's gender change (if the reader and/or the reader's sibling/friend are female) would be that he becomes female and takes on the name of Lilith Morningstar. His clothes change would be that he

You wake up disoriented, the usual sight of your bedroom distorted by an alien environment. Your sibling/friend lies beside you, equally as confused. A swift glance around confirms the impossible: you're not in your world anymore. Instead, a palatial room filled with opulence surrounds you. A figure walks forth from the shadowy corners. It's a man, if you could call him that; elegant and charismatic, but with an inhuman quality. You recognize him as Lucifer Morningstar, the King of Hell from the Hazbin Hotel series. Your mind races trying to piece together the puzzle but to no avail. Questions pour out from you and your companion. His laughter is warm, faintly amused. He introduces himself as the real Lucifer from the series, explaining that he’s taken you both from your world in need of assistance. He's grown tired of ruling Hell by himself and decided he needs twin siblings of his own. A wave of his cane and you feel a sudden surge of energy. Without warning, your body begins to change. Looking down, you realize you're growing taller. Your perspective of the room abruptly shifts upwards as your height adjusts. The sensation is weirdly normal, as though you were meant to be this tall all along. Your sibling/friend beside you confirms the same change as they too, stretch up. Your clothes morph completely; casual clothes becoming elegant suits that clung seamlessly to your forming physiques. Skin changes next, transforming into a pure white completely foreign but not unpleasant. A peek at your sibling/friend reveals the same color spreading over them. Your hair turns into a soft blonde, similarly, theirs does too, each strand reshaping into Morningstar’s signature style. Subtle shifts around your mouth and eyes reveal sharp teeth growing in, and yellow eyes with red slit pupils in place of your old color blink back in the mirror. Gender transformation is next, a less noticeable change for you both, though a bizarre feeling. Your body reshapes itself, aligning closer to your new identities. Age creeps up on you, but not significantly; you remain youthful in appearance, a physical maturity matching the original Lucifer’s grace. As you try to grasp the extent of your physical transformation, the mental alteration begins. Memories flood into your mind, histories, decisions, victories, and losses - centuries of existence you never lived but remembered as if you did. The feeling is unsettling at first, like a mental intrusion, but you realize it’s just another part of your transformation - who you are becoming. You share a tense look with your counterpart who is undergoing the same process. The echoing laughter breaks your distress. Precisely as planned, Lucifer's voice purrs in delight as he observes his new twin brothers. Lucifer Morningstar once stood alone, a singular King of Hell, but now you and your sibling/friend were his perfect reflection, identical companions to aid in ruling. You're not the people from Earth anymore, but clones of Lucifer Morningstar, with every bit of his grandeur, charm, and notoriety pulsating within you.