6 yrs. ago we had a armstrong kitchen floor installed in our new house. About 2 yrs. ago we noticed brown spots on the floor that we couldn't clean up. Had the installer come out and said it was the underlayment coming thru. Armstrong said it wasn't their problem. They also said we should of used a better grade (thicker) floor. We are planning on replacing the floor, should we also replace the underlayment? If not can we put the new floor over the old one? Thanks for any help.