If you fail to upgrade the AP2800 series, AP3800 series, or AP4800 series to Cisco Mobility Express version 8.8, you must first have an interim upgrade to Cisco Mobility Express 8.5.140.0, before upgrading to Cisco Mobility Express 8.8.
Alternatively, you can first load the 8.8 CAPWAP version (non-ME version) as the interim version before upgrading to Cisco Mobility Express 8.8.