The release notes of newer software releases will provide you with what bug fixes, new enhancements, security vulnerabilities (if any), Client Interoperability additions or fixes, etc were either added or resolved.
I invite you to go and download and read them on the support site which is always best practice before upgrading any piece of equipment or software.
For example, upgrading to version 10 of ZD code provides the following new feature sets.
Key features of release 10.0 are
Support for indoor 802.11ac Multi-User MIMO (MU-MIMO) Wave 2 AP R720
Increased scale for ZD 1200
Enhancements to Application Recognition & Control (ARC)
Deep integration with Cloudpath
Role Based ACL
Change of Authorization (CoA)
Spectrum Analysis for 11ac APs (Wave 1 & 2)
Modular software for APs
Support for 802.11w
Simplified Self-Service Guest workflow
GUI support for BSS Min Rate, OFDM Only, and Management Tx Rate