If you have above-average/exceptional rebounding (which I would say you do for the two post positions) you can get away with it. I've used my SF slot as a "point forward" for distribution of the basketball with success. I've also used it as a "shooting guard B" position (although if I'm doing thing, I usually try to make sure the kid has at least a minor rebounding total...even if it's just in the teens, it matters some).
I would try it out in the exhibition season and make sure it doesn't create any glaring problems, but my gut feeling would be, as gvsujulius said, that the benefit you get from the SR guard's IQ will more than make up for the learning curve and overall lower ratings the freshman SF would likely saddle you with.