You need to delete the license identifier. If you do a "show system license" the bottom section will show the actual licence keys installed. Look for the one that is for the AX and use the license identifier to delete the license. For example
Licenses installed:
License identifier: JUNOS825794
License version: 4
Valid for device: <serial number>
Features:
idp-sig - IDP Signature
date-based, 2016-08-18 17:00:00 PDT - 2017-08-17 17:00:00 PDT
request system license delete JUNOS825794
