be not written in stone

To not be permanently fixed or firmly established; to be able to be changed, ignored, or avoided. The deal isn't yet written in stone, but we're confident it will go ahead as hoped. Please tell me that rule is not written in stone—we would be a lot more productive if we didn't have to adhere to it every time. Keep in mind that these blueprints are not written in stone—they're just to give you a feel for the design.