Same thing for me. I assume the reservations we already have with the coupon are still valid, but I don't know for sure.
I got worried so I did the online check-in and completed the skip-the-counter process. All went well with getting the confirmation. My confirmation says to go straight to the vehicle and show my printed reservation confirmation to the booth agent. If I can't find a lower price before my May-25 arrival I'll stick to this and see what happens. I still have the option to cancel. My credit card will not be charged before my trip except possibly $1 according to Alamo.
I checked prices again and tried using the $35 off and the message said "Reservation start date must before Tue Mar 01 23:59:00 CST 2016". I guess they meant must be before that time but that's weird since we are way past that now.