I did a similar thing...My radio is on top and climate controls on bottom. I even had BFG make me an upside down cover plate so all the markings read right -
There are alot of screws (7mm or 9/32 or 1/4"). Some attach the frame to the dash opening (from the fire wall side), these are the tough ones...I think there are 6 (I think), 2 on the left 2 on the right and one or 2 on top. You can't see them, only feel them
There's more attaching from the bottom as well.
After you get all the screws out, the plate will be really loose but you won't be able to get it out. The easiest way I found is to fold the front down flat, than you can turn it and get it out.
When I did mine, I cut the "L" into 2 pieces and reinstalled them seperately. After they were both in and mounted, I hooked them back together with some brackets for added stability.
Looks cool works great. Why GM did the arrangement they did I'll never know.