You cannot open the workbook for the previous versions either u save the file as .twb or .twbx of version 10.
As far as I'm aware there is no way of saving a downgraded version.
You can install and run multiple iterations of tableau on one system though, so you could have a 9.2 version for running their workbooks.
Thanks Treveni for confirming what I thought was the case, and thanks to Chris for suggesting a work around. I see this as a big limitation for Tableau, and something that the sales rep failed to point out -- not good! Our organization will need to take steps to coordinate future releases in order for us to access files in a timely manner.
Also I noticed that Tableau does not automatically update when you are working on a file located on a SharePoint site. Unlike Microsoft products where a spreadsheet or document can be opened, edited and saved again, with Tableau you must save the workbook locally, make your edits and upload again to SharePoint (otherwise your work will save to temporary internet files).