User blog comment:Mathmagician/Installing Reborn Mod - Walkthrough/@comment-24.205.121.44-20120311145040/@comment-4674838-20120311233338

No, you shouldn't lose your world. Your world and character data should be backed up (like I said in step 2) so that you always have a copy of it. And you should still be able to play the mod with your world/char (see step 6).

And if you install reborn and then don't want it, you can always uninstall the mod and uninstall Terraria and redownload it through steam for free, and you should still have your worlds/chars and everything goes back to the way it was before you modded.