Per the 10.2 Server Upgrade Page | Tableau Software workbooks are not upgraded automatically when upgrading the server. However....
workbooks will be automatically upgraded when Web Edited on Tableau Server, the workbook's extract is refreshed on Tableau Server, or the workbook is saved in a later version of Tableau Desktop.
meaning most workbooks will likely be upgraded by users as they are being interacted with. This article has more details: Version Compatibility Between Tableau Desktop and Tableau Server