As I understand the rules as posted on the employee travel site, you can not make a change to your reservation within 21 days of initial departure. If you do attempt to change, the rules say you will forfeit the ATW coupon completely.
However, since they are one-way passes, it seems to me that you could non-rev on the outbound and use the return coupon as originally scheduled. The outbound and inbound trips are not connected to each other in any way that I know of other than your name and employee number, so there would be no way for them to cancel your return positive space pass just because you had to change the outbound leg which then got cancelled.
Last month I used two of my ATW passes. One to fly DFW-PHL-BUD to join a Danube River cruise. The cruise ended at Mainz, Germany (nearest International airport is Frankfurt), and I used the other pass to fly FRA-PHL-DFW. BUD to FRA is quite an open jaw on a roundtrip ticket, but no problem with these passes. I had to check-in separately for both, and on the outbound they asked no question regarding how I planned to get back home, nor did anyone ask how I got to Frankfurt. It was a non-issue to them. In fact, I got upgraded to First Class for the FRA-PHL leg. FWIW, PHL-DFW was a nightmare. Thanks to the TSA in PHL, I missed my connecting flight to DFW and had to fly AE to MEM, sit for 2 hours, then fly AE MEM-DFW. My checked luggage and I did not arrive on the same flight. It did make it to DFW, though, and I was able to retrieve the checked bag the next day.
Before forfeiting your pass, ask yourself why you can't leave until the afternoon. I would try to figure out a way to leave on the early morning booked flight. These coupons are worth their weight in gold (from a guy who got upgraded to F/C on an international flight.).