"Ian! C'mon, I'm waiting!"
You continue to pump your cock into the loose condom, resisting the urge to do as Mark says. "Just a minute!" You voice cracks right in the middle of the call, and you hope Mark doesn't decide to come to investigate.
As usually, you feel your cock changing to fit the condom first, growing to a fairly decent size in your hand just a bit bigger than Mark's you guess. But you already know you're not turning back into yourself, as you know the feel of your own cock and this just isn't it, the length is right but it's much thicker. Still, as your balls drop lower between your thighs as they grow to the size of chicken eggs, you have no choice but to keep going.
You sit down on the bed as you feel your body stretching out, shooting up from 5'10" to 6'2" in a matter of seconds, back to Jay's height and ball size, even if your cock is a little shorter, somewhere between Jay's and Mark's in size. You reach up a hand to brush shrinking but still blond hair out of your face as you loose Ian's tan, your skin turning paler and a smattering of freckles forming along your chest and shoulders. Glancing in the mirror on the closet door confirms some across your face as well which seems.. oddly familiar as your eyes shift from gray to green.
You begin to pack on muscle, first your stomach tightening into a six pack followed by your pectorals plumping outwards. Every pump of your arm as your jerk off makes your biceps look just a little bigger. Dark blond hair creeps up the middle of your stomach as your arms and legs darken from more hair growth, and a light dusting grows in along the middle of your chest.
As you cum, you feel an itch along your jaw as a little bit of stubble grows in, just enough you could probably make a go at a sparse beard if you wanted to. As you flop back on the bed to catch your breath you notice the room has changed from the guest room again to something actually occupied. There's the musky smell of sweat in the air, probably coming from the pile of sports equipment in the corner. The desk is piled with text books, and posters of cars, bands, and athletes cover the walls.
You peel the condom off your cock and throw it in the trash, moving over to the mirror to try and figure out who you've become this time. You look about eighteen or nineteen, your hair is still blond but in more of a short, slightly spiky cut. You lost the surfer vibe and gained a jockish frat boy one. Leaning in for a closer look of your face it suddenly hits you, you look like Scott with five years and a huge growth spurt. You look around the room quickly and find what you were looking for, a framed family picture on top of the dresser that was in young Scott's room, except instead of Mark with his arm around a younger's Scott's shoulder, its your new body in the big brother spot with an arm around Mark's shoulders.
"Hey, Scott, can I borrow, oh god!!" You jump a bit but look over to your open bedroom door to see Mark shielding his eyes from your nakedness.
"I told you to knock, squirt. It's your fault if you get an eyeful." Though you look around and find a towel to wrap around your waist. "What do you want?"
"Can I borrow some of your cologne? I got a date tonight." Mark still isn't making direct eye contact with you, as you put your hands on your hips. You think about just telling him to get out of your room so you can try another condom, but after two shifts in quick succession you feel a bit spent.
"Whatever, it's on the nightstand, I'm going to take a shower." You walk past Mark and head to the bathroom. This wasn't a bad body, maybe spending an evening or even a day in it wouldn't be too terrible.