Based on new firmware security against malicious images you cannot directly upgrade or downgrade from a Fully Signed (FSI) image to a Unsigned image (UI). You must downgrade to a Intermediate signed image (ISI) which bridges the fully secure version running on the r500 to the unsecure older version you are trying to run.
The ISI image was added to standalone code 104.x; ZD 9.13..0 and Unleashed 200.2
Best suggestion is to install the standalone version 104.x to the AP which is an ISI image. Then it should accept the older ZD code.