Are you able to replace the data source for each version of the workbook? here are a few resources that might help:
I hope this helps.
Just throwing this out there - I think you might want to go back to the drawing table on your requirements.
I'm not sure why you would want to store 'old' reports, it seems to me that when a client is looking for a report they should be directed to the most current information possible. THEN if they wanted a historical perspective, you could include a dashboard that shows a trend over time or similar. Using this approach you would not need to store a cache of historical reports, it would be inbuilt into your solution.