Today Blackberry Leaks reported a leaked version of BB OS 18.104.22.1688 for BlackBerry Curve 8520 which points that sooner or later RIM will be rolling out an official version of it. I tried installing it on my BlackBerry 8520 and it was successful without any issue and its working smoothly till now.
Once you run the setup it will do some installation and then you will have use your BlackBerry Desktop Manager to install the upgrade. As it takes care of backup your data you dont have to worry much about loss in case the installation doesnt work for you. It took around 30 minutes in total for the complete installation.
However its a leaked version so we warn that it might not work for everybody and we do not take any responsibility if something goes wrong. You can download a copy from here.