Hello David,
When the upgrade path is not followed ZD does upgrade to the uploaded image although as explained by Martin it would come back with Factory settings. The error image is invalid, not supported is seen on the AP's which let say are on quite old version and are connected to ZD with latest image.
Anytime you don't follow the upgrade path ZD would go to factory state, regarding the downgrade, ZD only supports one down image (that was available previous to upgrade) apart from that image if you downgrade ZD goes to Factory state.
Best Regards
Vineet