Although a planner booking inherits the associated key's description, you can edit the booking to change it's description. Furthermore, you can change which key a particular booking is associated with.
You can edit the booking by double clicking on it in the Planner window. This will bring up the Planner Booking property sheet.
Enter a new description for the booking and, if appropriate, select a new key with which to associate the booking. Press OK to save your changes.