Hey Straightarrow, I recently read an article in the "Bowhunter" mag. about this very topic and will break it down for you. A guy by the name of Randy Ulmer wrote this piece. He says that if you hunt more than 2000' above elevation where your bow was sighted in, you will begin to notice a higher arrow impact point, especially if your arrows have feather fletchings. He suggests, that bow hunters set up like this, by focusing on your longest pin setting, wether it be 40 or 50 yrds., set out a target and shoot until you're comfortable that you are executing good shots, then simply turn your limb bolts (reducing draw weight) in small increments until you are hitting dead on at this range. He notes, to be sure to turn both bolts equally to preserve the tiller setting. After making these adjustments, move up and check your short yardage pins, and he notes that you may actually have a slightly lower impact point, but that it should be insignificant.
Hope this helps ya. It does make sense to me, but I've never hunted high enough that I would have to make these adjustments myself, so I'm pretty much giving ya info. that I've read. GOOD LUCK!