With changes to memory layout and internal processes, the ZD upgrade path has long been needed to retain existing/earlier configuration when upgrading. We always recommend saving a backup (and updating the file name to remind you what version it came from), at each upgrade, which allows you to restore the confugration if you need to revert back for any unknown reason.
Without a saved backup, a ZD downgrade can result in a factory default config (also an option
if you want it).
If I guess your situation, you maybe upgraded to 9.10 and configured WLANs but have APs that
aren't supported past 9.8? The Support site, under Tools, has a Log Analyzer (Rowdy) that can
read a ZD debug (save one from Administer/Diagnostics). You can view your present 9.10 WLAN
details, to help recreate them if you have to start over after a 9.8 downgrade.
If you have APs which did connect on 9.10, they will retain their AP names (but not location) info
after they reconnect to your ZD on 9.8, which might help restore them too. I hope this is helpful.