After the upgrade, check if the tags were moved under "/var/upgrade/PreUpgradeBackup". I have seen this in the past. If tags are indeed moved there, you can move back the tags to etc.
During the upgrade AEM will attempt to backup customizations and store them beneath /var/upgrade/PreUpgradeBackup/<time-stamp-of-upgrade>. In order to view this folder in CRXDE Lite you may need to temporarily enable CRXDE Lite.
The folder with the time stamp should have a property named mergeStatus with a value of COMPLETED. The to-processfolder should be empty and the overwritten node indicates which nodes were overwritten during the upgrade. Content beneath the leftovers node indicate content that could not be safely merged during the upgrade. If your implementation is dependent on any of the children nodes (and not already installed by your upgraded code package) they will need to be merged manually.
Check [1] for more details
[1] Post Upgrade Checks and Troubleshooting