Instead of swapping with the people you saw in the street, you decided instead to go and grab new parts at the local gym you went to. There, you could swap with people who had toned and treated their bodies right, not to mention you could swap with the hot girls who frequented the gym.
You quickly donned some workout gear; shorts, shirt and running shoes, hopped in your car, and drove to the gym. Once you arrived, you head directly for the treadmill area where you would be able to observe most of the gym and pick out people for you to swap. You set the machine to a light walk so as not to look conspicuous and set your sights on somebody to swap with.
You decided that you wanted to swap with the females of the gym. Swapping with the males sounded okay, but given your new-found abilities you thought it would be much more exciting to try out the other side of the gender divide.
You saw in front of you a women who would make for a great candidate. She had the body of a runner, with shapely and powerful legs. She was tall and fair skinned, with short cut blonde hair and small breasts. Your first thought was that you wanted her head, but since you could not directly trade body parts you had to find some kind of work around. You wanted to eventually have an entirely female body to work with and hers seemed like a great start...but you had to swap unusually.
Then an idea struck; if you had to swap abnormally, couldn't you just swap two of the same parts in an odd configuration? Hoping against hope, you attempted to swap heads with the girl, only you didn't swap them as one would think. Instead, mere moments after attempting the swap, you become disoriented and almost trip in the treadmill as you can now see down your back and your butt, and now able to see the backs of your feet as you walk shakily. You reach up and try to touch your face and must reach behind you to touch your head. To your right you can see your left hand, and the same on your left. Your hands cup your soft cheeks and you realize your success. Fumbling about, you manage to press the STOP button behind you and awkwardly turn around to look at what has become of the girl. In front of you, you see the runner, jogging casually on another treadmill, only your head is on backwards on her athletic frame and she doesn't appear to be panicked or hindered by it at all.
You were pleased with how it had turned out and son implemented the second part of your plan. You swapped everything below your and girl's neck, only they were backwards to your heads, and now you and the girl had both swapped bodies completely. You even got her clothes too, which was likely a good thing given that everything your male body had been wearing would have been at least a size too big for your skinny female body, not to mention your shoes.
Now, however, you admired your new body. You were in excellent shape, it appeared, as you snuck a peek at your toned abs and flexed your toned arms. The only thing that bothered you were your new barely B-cup breasts.... but you could fix that!