So you're going to upgrade your Actuate iServer implementation. After you read this, you might consider moving to the 'Cloud' with your deployment. Then again, it really isn't to difficult to do but you'll need to know how to apply version specific jar files and recompile.
On the topic of moving your reporting enterprize to the 'Cloud' check this article, where Actuate's CEO talks about being architected to 'move from the warehouse to the cloud'. http://www.v3.co.uk/v3/news/2273651/actuate-birt-open-source
Now the secret to upgrading the iServer RSSE... Really it is a matter of first simply extracting your iServer installation and deploying the iServer. Then you will have a folder named /acrsse/. Here you'll find a few example Security Extensions. Pick the one that you have implemented for your custom RSSE, then pull all the bin/jars out and use those in your RSSE project.
Compiling against those, and changing the version number in your configuration class will give you the new updated RSSE, provided something doesn't happen to the foundation classes... As it sometimes does; just read the release notes if you don't want to do a bunch of needless troubleshooting.