I don't believe theres a shortcut for directly replacing a material with something else. the closest to this is ctrl+shift+click but it can only replace air with materials and materials with air, and when you want to for example replace TUNG with TTAN and the TUNG is surrounded with air you have to ctrl+shift+click the TUNG into air and redraw in TTAN because floodfilling with TTAN will flood the unrelated air around the TUNG. please add a shortcut for directly replacing materials with another if you can.