You cannot remove you old tasks untill and unless you deleted the old tasks from old document.
In order to Remove old tasks from old document to new document.
Follow the steps mentioned below.
1) Make sure to have one application with old application name in the same folder.
1) copy all the tasks from all documents and paste it in new document.
2) delete old tasks from old document.
3) edit the copied tasks and change names of tasks properly.
4) now you can remove old application.
By this you are up and running.
Hope it helps