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I have Videostudio 9 as full version and 10,11, and Pro x2 as
upgrades. I'll be getting a new computer in a few weeks. I don't
want to install all versions in my new computer.
Which versions do I need to install in my computer so that
Pro x2 will run? Maybe just VS 9?

I think that you only need the serial number from an earlier version.
Try running the VSX2 installation and it should ask you for
a. The Serial Number for X2
b. The Serial number of an earlier version.