I do not know if you have figure out your problem yet, but to reverse a closed WO, you first need to go to WO-A and change the WO status to "R".  Now, go to WO-I.  Do not enter a date, but select the "Lookup" button lower left of screen.  Find the WO you want to reverse, highlight and then double click.  Now press "Enter" and you should be prompted to reverse the WO.  Note, if you deleted the UOH for this assembly, this transaction will create a negative UOH. You will have to correct after reversing.
 
Once reversed, you can now go to WO-J and "Cancel" the WO.  This will return all the material components back to stock and put any remaining labor to WIP Variance.
 
Let me know if any problem with this.