You interject:"Seriously, I did not seek not invade your homeowww--" when you suddenly notice your speech going odd. "Oh, don't worry, soon you'll be a permanent resident here", she cackles as she watches your face stretch into a feline muzzle complete with fangs and whiskers. Spreading up your head, you can feel your ears warping into a pointy shape, suddenly making your hearing more sensitive than it was before - as is the case for sight through your nightvision-enhanced eyes. You shiver in horror at the sudden warm feeling of fur covering your body, a glossy coat of black fur rendering any clothing unnecessary.
You try to make a run for it and exit back through the door, but suddenly you lack the manual dexterity for it. Your fingers have stiffened, and quickly shrink together with your hand palms into small black paws, adorned with retractable claws at the end. Your feet haven't fared much better, as you are now able to effortlessly shake off your socks and shoes, revealing your hindpaws. The lack of balance makes you fall over onto all fours, much to the witch's amusement.
With most of your feline body done, the bone shifting starts and your spine straightens itself out for walking in a quadrupedal position, and your arms and legs become real short in comparison to your body. The changes almost done, you feel a ripple going down your spine and finally pushing out your back, slowly producing a black cat tail that coils itself in confusion. "And now for the icing on the cake...", the witch grins.
Your feline body complete, you feel a strange sensation in your gut as your clothes begin to cover you. Once the sensation stops, you cautiously step out from underneath them, reduced to a normal cat. "You'll make a fine familiar", she says.