Minedeas: Version changer

I think there should be an option in Minecraft where you can change the version instantly. Instead of messing with .jar's, they should just have one big .jar with all the versions. This might sound huge, but some updates don't have many changes, so you would compress all the duplicate stuff. And to prevent world corruption, after you set the version in the options, you go to singleplayer and it will show a menu with options for 1.4 worlds, 1.3 wolds, etc. So if you want to mess around with previous version corruption, you can, but it won't force you to have to remember world versions if you don't. It would also show the world version under the world save, along with the survival/creative and cheats place. This is my idea, and i hope you like :)

P.S. I drew picture myself, not copied from anyone.