After opening an implemented design, going into the device view in either PlanAhead or Vivado Integrated Design Environment (either clicking Ctrl+U, then selecting a used location, or clicking the location and using the hot-key shortcut Ctrl+U) fails to unplace the location. Right-clicking and choosing the option works, and indicates that the hotkey shortcut CTRL+U should accomplish the same behavior.
Additionally, if you right-click to get the pop-up menu, then select Ctrl-U, the object will be unplaced (you might as well just finish it off with the mouse).
Both Ctrl+U and Ctrl +W (Swap) require the context menu to be open before they are active. The other shortcut keys shown in this context menu do not require the context menu to be opened first.
In Vivado Design Suite 2013.1, the Ctrl+U (unplace) and Ctrl+W (Swap) are not active until the pop-up menu comes up.
This requires that you right-click the component(s) and unplace or swap through the menu box.
This issue has been fixed in Vivado Design Suite 2013.2