I booked a return trip today using the AARP discount with the $400 reduction. It was only after seeing Hobbes01's post that I decided to take a closer look at the terms and conditions. In it is:
"Discount does not apply to open jaw journey (only round-trip with same origin and destination permitted)."
The flight I booked today was indeed an open jaw (JFK-LHR-LIS-LHR-PHL-TPA) yet the $400 discount was applied without any problem.