A few principles should be kept in mind when determining the impact of switching a backup item's policy from Policy 1 (P1) to Policy 2 (P2) or of editing Policy 1 (P1).
All changes are also applied retroactively. The latest backup policy is applied on the recovery points taken earlier as well. For example, assume that the daily full retention is 30 days and 10 recovery points were taken according to the currently active policy. If the daily full's retention is changed to 10 days, then the previous point's expiry time is also recalculated as start time + 10 days and deleted if they're expired.
The scope of change also includes day of backup, type of backup along with retention. For example: If a policy is changed from daily full to weekly full on Sundays, all earlier fulls that aren't on Sundays will be marked for deletion.
A parent isn't deleted until the child is active/not-expired. Every backup type has an expiration time according to the currently active policy. But a full backup type is considered as parent to subsequent 'differentials', 'incrementals' and 'logs'. A 'differential' and a 'log' aren't parents to anyone else. An 'incremental' can be a parent to subsequent 'incremental'. Even if a 'parent' is marked for deletion, it's not actually deleted if the child 'differentials' or 'logs' aren't expired. For example, if a policy is changed from daily full to weekly full on Sundays, all earlier fulls that aren't on Sundays will be marked for deletion. But they aren't actually deleted until the logs that were taken daily earlier are expired. In other words, they're retained according to the latest log duration. Once the logs expire, both the logs and these fulls will be deleted.
With these principles, you can read the following table to understand the implications of a policy change.
Old policy/ New policyDaily fulls + logsWeekly fulls + daily differentials + logsWeekly fulls + daily incrementals + logs Daily fulls + logs-The previous fulls that aren't on the same day of the week are marked for deletion but kept until the log retention periodThe previous fulls that aren't on the same day of the week are marked for deletion but kept until the log retention period Weekly fulls + daily differentials + logsThe previous weekly fulls retention is recalculated as per latest policy. The previous differentials are immediately deleted-The previous differentials are immediately deleted Weekly fulls + daily incrementals + logsThe previous weekly fulls retention is recalculated as per latest policy. The previous incrementals are immediately deletedThe previous incrementals are immediately deleted-