Are you needing to take the inactive projects off your system system entirely to recover disk space? Are you setting up v11 on a new environment that has less disk space, so you can't move the inactive projects to it?
Another alternative is to take a backup of the project's database, and a backup of the project's Repository directory. You can paste those into your v11 environment, make a few modifications to the dbid.xml file, and then use Site Admin Restore to bring them into your Site, and then use the normal Project Verifcation, Repair, Upgrade process to bring the up to v11.
(Opinions and information contained in this post are wholly my own and do not reflect the opinions of my employer.)
This has always been a problem with QC. You must keep data current or lose the ability to quickly retrieve it. For example if you are on QC 8.x you would have to install a 9.x system, do a migrate, install 10.x system, do a migrate, install a 11.x system, do a migrate.
IMO - This is unacceptable. HP should provide backward compatibility for archiving projects.