The cashier led you into the backroom, where you had to strip naked and let the medbots give you one last human checkup before you physically lost the humanity you'd already given up legally. You thought the old man was going to step aside and give you some privacy until it was time to change, but he had to stay and be a witness that every stage of the process had been successfully completed on the same person throughout. Taking off all of your clothes in front of a man you didn't even know was by no means an everyday occurrence for you, but he only said you'd better get used to it. Modesty isn't something you're going to be entitled to from now on.
After the examination was over and a medbot injected you with something, the old man led you into a chamber with a giant light overhead. "Now be a good girl while I go warm up the morphic emitters. Stay," he said, already talking to you like an animal. He locked the door, through which you could still see each other, and punched a few buttons until the overhead light came on. You felt an electric tingle go through your body immediately and moaned.
Seeing your reflection in the door, you noticed that your wrinkles were going away and you were losing weight. You looked younger than you head in a decade or more! Your boobs were firming up and you looked just like you had in your old college yearbook. You heard the old man complain about age regressions only being allowed for people who wanted to give up their humanity. If the transformation stopped now, you could start your adult life over, hit some bars and get some hot dates, and maybe marry someone better than your first husband turned out to be. But your transformation wasn't going to stop now, and you knew that.
As the technology kept on stripping off the years, you noticed yourself starting to become very obviously shorter. Your breasts shrank, your face got chubbier and your body hair started to thin out. Puberty had taken years to go through, and now it was taking only minutes to reverse. You suddenly got a lot more energy and felt like bouncing around the chamber. If the transformation stopped now, you'd know roughly what to expect. It's a good thing real kids don't know what awaits them in adult life! You hope growing up the second time won't be nearly so miserable, since you'll be doing it as another species.
By the time you look like a toddler, you're somehow managing to shrink even more. But this time, instead of looking younger, you're looking fuzzier. White and brown fur is sprouting all over you, and your nose is turning black and starting to feel wet. Getting a dog's nose in a pet store is torture: you're overwhelmed with not only the scents from yourself and the old man, but also lots of different animals of dozens of different species, plus lots of different kinds of pet food that are starting to make your mouth water. The heat of the chamber causes you to open your widening mouth and let your pink, floppy tongue start to hang out. Your little panting muzzle is adorable!
Your ears are starting to look really flat as they grow longer and wider, until finally they flop over and their triangular tips hang down over the sides of your face. Your reflection reminds you of the puppy your dad brought home for you and your sister when you were six years old. Surely some family is going to think you're just the sweetest! The thought of being fawned over and adored makes you smile--except smiling feels different now. It feels like a muscle spasm in your butt, waving around a significant portion of your backbone back and forth. Turning sideways and checking your reflection from that angle, you notice a long (well, long for your tiny body, anyway) brown-and-white tail wagging in a fast side-to-side motion behind you. You yip loudly, and your tail wags faster. A mischievous feeling builds up in your playful young brain, and you turn around to try to catch it. Chasing your tail only makes it wag harder, making your little game all the more challenging, all the more fun to try, and all the cuter to watch. Even the stodgy old man giggles at the puppy-like scene in the chamber.
Catching your tail makes you realize just how soft your fur is, and how hard it's getting to hold things in your hands. You let go of your waggable accessory and put your hands up toward your face to look at them. Pink pads are forming on the bottom, and fresh little puppy claws have replaced your nails. Your legs give a sudden jerk, and you're all at once pulled up on tiptoes, making you fall forward onto your still-forming forepaws. Your insides gurgle, and with a few more skeletal changes its all over. You bark at your reflection again, thrilled at the heart-warming sight of yourself as a cute little St. Bernard puppy. You wish you could stay like this forever, but you're aware that dogs grow up fast compared to humans. You hope your doggy adulthood will be better than your human adulthood, at least. How bad could it be?
The man is opening to door to the chamber, when you suddenly feel the strangest sort of warm trickling feeling behind you, after which you also hear and smell it, causing you to look behind you. A yellow puddle has formed and is spreading across the chamber floor. So much for being house trained! You really do have a lot of growing up to do as a dog! The old man picks you up and cusses about having to mop the chamber floor again before carrying you out to where the other pups are.