You could establish two data source connections to the same Excel file: one where the connection is set to 'live' so that adjustments to the file are picked up automatically, and a second instance where you create an extract of the Excel data so that you are 'locked' to one snapshot of the data at a given point in time. You can find more information on how to create and work with extracts on Tableau's Online help page about extracts: Extract Your Data
Simply establish a new data source connection within your existing workbook as such:
You'll see two entries in your list of data sources on the top left of the user interface.
You could simply rebuild your sheets using fields from your extract variant of the Excel data source. This way, you can compare sheets based of live data versus data last refreshed 'x days ago'.
For instance, here's a sheet where I use the dimension "Status" on my Excel data source set with a live connection:
And here is a second sheet using the same dimension from the data extract:
If you have a Tableau Server environment available, you can place a refresh schedule on that data extract.
Let me know if this answer helps!