The first thing you would need to do is find the 4.2.0 software. I have the 4.2.0.67 version of the software that you can download from here:
vB Easy Archive - Rogers BlackBerry Pearl 4.2.0.67 OS Update
As far as downgrading you can find those instructions on BlackBerryFAQ.com:
How do I downgrade the OS on my BlackBerry? - BlackBerryFAQ.com
Update: looks like my site is having issues. I will update this again once it is available. You can also search around to see if you can find this version of the OS, or any other 4.2.0 software.
Update #2: I posted a different site for the download.