It's a common question amongst the developers - how to change package of multiple reports in one go! Unfortunately, Cognos doesn't have any built-in facility to do this at the moment.
However, there are some work-around for this.
1. Use third party tools: Some private tools hook-up to Cognos Content Store and allow you editing multiple reports simultaneously. This includes changing the report package, access permissions, visibility, etc.
2. DIY: In this simple Do It Yourself method of doing bulk update on reports, I place all the reports in one folder. Then generate deployment archive of the folder using Export wizard. This pushes all the report definitions in one xml file. Pick up the deployment archive, unzip and examine the xmls.
Once you find the xml that contains all the reports, open it in a good xml editor. Now, search for the package name and identify the pattern you need to replace.
Now, simply replace all the instances of package name using that pattern, put the file back in zip and bring it back in Cognos using Import wizard.
Voila, here you have all the reports back with the Report Package changed to the desired one.
Note: While importing, you might want to change the 'destination' of folder so that it creates a new copy of reports. So, in case of any issue, you still have the original folder intact.
I hope this helps!