Last fall, I ran a 2:38 at Chicago. Because of my schedule and then a back problem, I had to back off my mileage. While I was coming back, I tried various things to see how my back felt. (I suspect that I had a piriformus or similarly located problem that pressed my sciatic nerve, which set off other problems.) These things included running barefoot on the grass and running in flats without my orthotics. During these activities, I naturally landed midfoot, rather than heel. It felt more natural although a little more fatiguing, but within a week I never went back to heel striking, and heel striking feels awkward now. I have no idea how this will affect my training and race performance because I haven't been able to press things in the last few months. I hate to give you a simplistic answer, but the bottom line is that I just did it, and it took about 2 days to change, and I'm still working on the form.