Here are the insrtuctions for downgrading your OS:

How do I downgrade the OS on my BlackBerry? - BlackBerryFAQ

Previous versions of the 8310 softwre can be found here:

http://www.blackberryforums.com/gene...ilanguage.html

Go to this section, and click the link in the first post titled "previous versiosn of OS are found here" until you find the version that you need.

Interesting that your carrier won't allow a verison higher than 4.2 though...