Before you upgrade your Symantec ATP from 3.x version to 4.x version, there are few things you need to be ready before perform ATP software update to avoid any hiccups.
*Note
- No fallback plan may cause redeployment to occur
- Lower version of ATP does not have the capability to restore the backup
- Read the release notes for 4.x version
- Take pointers
- Setup Backup at Symantec ATP
- Backing up ATP as a fallback plan
Hence, this overall process of the update may take an hour or 2 hours.
Reference