Ravi,
Technically the answer to your question is no. The only way it would be possible would be if you have unmigrated QC10 versions of all of the DB files (to include the QCsiteadmin_db and all project DB files) and the project repository. You would then need to uninstall everything and then install the QC10 installation using the QC10 installer (if you would still have it, as that version has been discontinued/sunsetted by HP for sometime and is no longer provided for download) and make sure that all files are replaced with the older non-upgraded versions. The thing is that you would then also lose all of your updates and changes made since you had upgraded your projects during the migration from QC10 to ALM11.

I believe that you probably have your reasons, but I am unsure why you would want to downgrade, as ALM 11 is basically discontinued/sunsetted as well and will also receive no updates or patches for the future, meaning all current OS, browsers, and security patches will not be instituted into either QC10 or ALM11 or supported (not to mention that all support for these versions has been basically discontinued as well through HP).

So essentially downgrading is not and has never been possible (and this is the case with any version movement new or old throughout the history of QC/ALM). Once a project has been upgraded to a newer version there is no way possible to undo that upgrade and roll back to a prior version.

Hope this answers your questions,
Dan