You should probably look at saving the workbooks with a new name and then move them to an archive folder, and then overwrite your original workbooks.
Are you using live connection OR Tableau extract ?
If it is live. Dashboard will have most updated the data always. If it is extract dashboard will not be refreshed until you do it manually.
Hope it helps.
Thanks a lot for your reply.
I'm using a live connection, as the reports are updated weekly, but at the same time we need a record of the data for previous weeks too.
Thanks Hari !! Let me give that a try.