I am also at a large program. we have lots of examples of people switching back and forth. one person started out AP only and finished up CP only last year. i'm not sure when he decided to switch and if he ended up having to do an extra year to complete CP b/c he had done so much AP already. another person started CP only, completed that, then decided to do AP as well. it took him 5 years to finish up.
We usually have 1-2 CP only and 1-2 AP only or AP/NP that start out in a given class, the rest are usually AP/CP. What ends up happening most time is the AP/CP drops the AP part and become CP only during their first year. in that case, we just ask that you finish up the rest of the year that you were scheduled to do so we aren't scrambling to cover rotations. We do have a few examples of people dropping CP to become AP only, and depending on their timing, they may have to do a fourth year. one person in this case got a Derm path fellowship, so he prob won't need CP anyway.
We have a mix n' match type approach to schedule, with the goal of 6 months of each, but in some cases we have to schedule people for 9 months of CP their first year (which is what I did, no big deal). you can imagine that if you make this decision early on, you can probably finish up the "-only" in three years, but you may lose a fair bit of elective time.
I agree with caffeingirl, you should probably do AP/CP and decide after taking a few months of each. Of course, this may be difficult if you are at a program where people do one for a year straight. I think they usually start w/ a year of AP, so you could probably finish the CP requirements during year 2 and 3 if you switched, i think. Also keep in mind that what you think you like and may end up doing could change after you get into residency and that having AP/CP makes you more flexible rather then likely being tied to an academic center as many of the "-only"'s can be.