After upgrade to Parallels Desktop 8 or 9 you cannot change the Release Input shortcut in Parallels Desktop ► Preferences as the function is greyed out.
CTRL+ALT is the default shortcut for Release Input in Parallels Desktop. It has been secured for this function in order to avoid confusion.
In order to assign different shortcut for Release Input function, please open System Preferences on Mac ►Keyboard ► Shortcuts ► App Shortcuts ► Parallels Desktop and add a new string for Release Input:
If Parallels Desktop is not present in the list just click on '+' button and chose Parallels Desktop in the Application drop-down list.