I really don't know step by step what will happen, but eventually it will kill them. Parasites will develop and eventually the water or the disease will destroy them. I personally change my water twice a week.
I have heard of heavily planted tanks where the nitrate is consumed by the plants, and since thats the end result of fish waste fish accaully can survive without water changes. but I have never heard of that with any type of piranha. the produce too much waste.
so my answer to your question is yes, they will die