As dannac says. In fact, whenever I have a layer I'm pleased with I make a duplicate and use "lock all" to keep me from making accidental changes to the original. Then I am free to try new things on the duplicate. Also, I make duplicates of a project as I work by using "save as" with the name of the original plus an identifier for that particular attempted change of the project. That way I can find an earlier layer from an earlier edition as needed.