As long as you don't cut the tier's into the soil but instead add it on top of the existing dam wall. I fail to see how it could cause a huge issue. Wouldn't it be in a round about way strengthening it?

Keep in mind I am not an engineer but when in doubt you could always ask Mr.Otto. His add can be found in the Pond Boss magazine and I believe on this website somewhere as well.

If you disturb the dam wall that someone worked hard to compact and sculpt...then I see horrible things possibly happening.