You've always been a fan of the New Zealand rugby team, so you decide to try on the All Blacks kit just for a laugh, since you know it won't fit.
In the large changing room, you take off all your clothes to put the rugby kit on. You decide to go commando because you love the feeling of the nylon shorts against your dick. You pull on the shorts which are, of course, far too big so you have to pull them up every now and then. You pull on the shirt which is also massive on you and then put on the socks and black rugby boots. As a joke you pick up the rugby ball you took with you and throw it in the air. When the ball lands back in your hands, however, it starts to feel very hot so you drop it in surprise. The heat stays in your hands, and you think they look more worn and calloused somehow. Suddenly, your fingers get thicker and longer, and your hands expand and widen. You can't help but stare as the heat spreads up your arms, thickening your skinny arms to massive proportions. A Maori-style tattoo pokes out of the skin of your right arm, as if it has been freshly done. This prompts your biceps to swell up, stretching the sleeves of your shirt. The heat continues to spread into your shoulders which widen and bulk up as your pecs balloon until you can't see your toes. You squeeze one of your new pecs through the now straining shirt with your massive hand, making you moan and giving yourself an erection. Your neck thickens with muscle and your chin becomes a square shape as a couple of your teeth fall out of your mouth onto the floor and your nose crunches itself out of place.
You see some hair fall on the ground as your hair is shaved away on the sides and back and reshape a itself into a trendy haircut. The heat continues to spread down to your abdomen, making a thick set of abs pop out and your back muscles expand. You shove your hand into your shorts as the heat moves to your crotch and makes your cock as thick as a soda can, and stretches it to 11 inches long. You rub furiously, wanking off to your own image in the mirror, and eventually spray cum all over it. The heat moves down and swells your thighs until they are thicker than the rugby players in the posters, stretching the tight rugby shorts to their absolute limit. Your calves thicken as well, to the size of rugby balls, tightening your socks around them. Finally, your feet stretch and grow until they feel strong and thick, filling out your rugby boots. You admire yourself in the mirror, and grin at your snake-like cock fully visible through your tight shorts. You open the changing room door to find yourself in an empty locker room, your new brain recognising it as the All Blacks locker room. You realise that you're late for practise and grab the rugby ball that started your change, running out onto the field.