Yes, if they are pyramiding it is dangerous, it puts stress on the internal organs and is a sign of an improper diet. The extent to which it can be reversed depends on the turtles age and how severe the problem is. The best thing to do now is to correct thier diet. Read over this:
http://www.redearslider.com/index_nutrition.html if you haven't already.
A few questions:
What have you been feeding them, how much, and how often?
What is your setup like? Do you have UVB lighting for them?
How big and how old are the turtles?
Can you post a picture of the problem?