This happened to me recently and having to solve the problem myself I thought it would be useful to share the solution.
Firstly, you can remove the door by loosening the screws without removing them completely.
In order to gain access to the plate you need to remove the oven. Switch off the oven at the mains. Remove the 4 screws at the corners of the oven. Remove the panel from the back of the oven. remove the two screws attaching the oven to the oven frame. This allows the lower oven to move forward to release the threaded plate. loosely screw the hinges on but do not attach the door yet as you will need access to screw in the 4 oven screws.
Peter McMahon, December 2007