Changing the peep could cause a very slight arrow flight because of the weight difference in the two peeps. It shouldn't even be noticable. Are you holding your riser or cradling it in your palm. You may be "tourqing" the bow causing the group to spread. I have seen people do this, trying to get used to a different and new peep. A new peep changes your optics. You aren't used to the veiw you now have. Try paper tuning the bow and see if the arrows are flying right. Stand at 10 feet and shoot 5 arrows through paper and see if the pattern is all the same. If they are, correct the problem, if not you are holding the riser too hard, loosen up. Hope this helped