This mod does pretty much what it says, it can change any world you've created or downloaded into a creative world or a survival world. Once you download it, it will have your first world already selected in the box but you can change it by pressing the downwards arrow and then selecting. If you want to change your world to survival or creative just click the box if you want if you want to have a creative or leave it blank if you wanted it Survival! It's very easy to do, so common sense should tell you really how to work it.
(This mod can be used for all versions of minecraft)