You dont want to go back to 4.2. Which OS did you install? You should just try another 4.5 build
I installed the latest OS for the 8320 that is stickied in this forum.
I downloaded a Rogers build (my provider) of the 4.5 OS from the Official BB site. When I try to install it, it says nothing needs to be done... Im guessing its the same..
Go to Update Apps on DM then click on advanced. Click next and the screen will show you that a downgrade will be done. Click finish to start the process.
Posted from my CrackBerry at wapforums.crackberry.com