If you view your workbook on Tableau Server, you can select the check box and then the Move link will appear in the Actions line. You can select Move and move a workbook and all its views to another project you have (already) created on Tableau Server.
Note that this does not impact the url address for the workbook/view on your server. Also, note that if the workbook is uploaded again from Tableau Desktop, the Project selected there will override the project movement done on Server.
server move.png 90.7 KB