I have a Sony A7R III. The small glass in front the the mirror box has cracked. I would like to replace it. I have looked up on youtube how to replace the mirrorbox. What I found so far is a video based on the Sony A7R II. Based on that video the mirrobox can be removed without removing the back plate of the camera and opening the main body. I have tried to do the same however the small cover keeping the mirrobox is stuck and I can't remove it.

My question:

Is there anybody here who opened up a Sony A7R III? Do I have to open up the frame of the camera and remove the back plate to remove the mirrorbox?