Try deleting the old one (twice) first and see if that helps.
- Right click on book in ADE and choose 'Item Info' to see where the real .epub file is.
- Right click on book in ADE 'delete from library'. (to delete from ADE)
- Delete the real .epub file (as identified in step 1) using file manager (Windows Explorer/Finder/...) (to delete real file)
- Now open the new .acsm file again.