Issue with Monthly Trend Report Filtered by Group

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I'm really just looking for some best practices here...

We have a few reports used primarily by our Contact Center that show monthly totals and trends.  Currently, the reports are filtered by Group.  Periodically, staff from the Contact Center will transfer to a different department.  This requires their staff account profile and group to be updated.  The issue is when they switch departments and we update their staff account, all of their previous totals that were included in the reports are gone.

I understand we can always filter by individual staff accounts, but that can be excessively time consuming to remember all employees who have worked in the Contact Center, especially when the report calls for 12+ months of data.