not be carved/etched in stone
From Longman Dictionary of Contemporary Englishnot be carved/etched in stonenot be carved/etched in stoneused to say an idea or plan could change John has several new ideas for the show, but nothing is etched in stone yet. → stoneExamples from the Corpusnot be carved/etched in stone• John has several new ideas for the show, but nothing is etched in stone yet.