You can format a SD card to be external storage again, however, you cannot leave the files on it then. Once it is made internal storage what is on it is encoded. So only that device it was in can read it and access what is on it. If you format it as external, you would either have to remove the items first, putting them on the phone or elsewhere (that won't work with apps downloaded directly to the card) or just format and lose all that is on the card.