How to revert back to older Unity3d Pro licensed version 3.4?

I had Unity 3D Pro. Version 3.4 installed on my PC. I wanted to upgrade it to the latest. But I accidentally installed the latest version 4.1 on top of it instead of requesting for an upgrade online. How can i recover my older Pro version so that I can upgrade later with the serial number of older version?

I think its not possible to revert to the previous version of Unity3D. You ll have to uninstall the current version and reinstall the desired one.

But I do not have the serial no. or license of the older one. It was already installed in the PC that I was provided to use. So is there a way to recover atleast the serial no. of the older one?