what you need to do is...
1. make a new world in don't starve together
2. open that world cluster which is on the world selection scree.
alternatively you can find the cluster in the game files
1. go to steam and click properties which you can find under the gear on don't starve together home page
2. press "browse local files" and with enough searching you can find that world there
3. there will be at least two "klei" files in your hard drive
4. with two drive tabs open you'll need both of them to have the "klei" files open in the cluster_("any number") selection place
(to find out what world it is there should be a little paper icon with a gear in the bottom left corner. double click it and all the info will be there)
5. take you pre existing world and drag them to the main don't starve game file and you have them all back
i apologize if you didn't understand a word i said
i forgot to mention, make sure to make a backup of your files, one time i did this and it deleted the world. thankfully it was a day 5 world but still.