But besides that, it is still RUDE to go in AP/CP with the intention of dropping CP later down the line. You are messing up the scheduling, creating more work for the chiefs, as well as your fellow residents who will have to cover whatever services you were suppose to cover. So if you know you want to do AP only, then go in to residency being AP only.
Meh. I guess I only agree if you mean someone would start residency, and that first day decide to drop CP and have the staff and chief redo the entire year's schedule. I doubt that every really happens.
If you are AP/CP and during the 2nd year decide to drop CP (after the year), congrats- you've just wasted all that time you already put into CP during residency. Plus, depending on your schedule, you might not have had sufficient time in AP service to qualify for the boards and may need that additional year to finish. If you do the same during the first year, then the program may have a slightly more difficult time filling that CP spot for the following year (if it's even necessary). Chances are, regardless of when you decide to drop, you would finish that year's already determined schedule- not require everyone to accommodate to your whim.
However, if your program is split AP then CP, and you decide to drop CP during your AP training, you probably don't even hurt anyone since the program can just try to fill that CP vacancy with another CP resident. From what I've seen, few programs have this set up, and those that do can afford to lose that CP resident anyway.