Does negotiating Credit Card Interest affect your credit?

Answer 1

if handled improperly, yes.

Answer 2

No it does not. All the credit rating agencies care about is weather you pay your bills on time or not. They are not concerned about the terms and conditions on which you have on the liability. You can negotiate any interest rate you want. If you are uncertain about the responses you receive to your question I encourage to check your credit report with any of the three rating agencies. The information they keep only reflects the date of the liability, how much and weather you paid the liability as agreed upon. They don't care weather your paying 3% or 21% that's your problem.