Got this documents from Netbackup Forum 2013, very helpful
Netbackup change retention policy question: If I change the retention attributes on an existing policy, do the retention attributes apply to that which was backed up prior to making the change? e.g. if I change retention attributes from 1 year, to 45 days, will data older than 45 days expire out naturally. Conversely, if I change the attributes from 45 days to 1 year, will the data the data that's been backed up under the 45 days day policy attribute be retained for 1 year upon change.