I have MS 11 installed on my PC. And today, I bought an upgrade to MS 13 Platinum. Will I have to uninstall MS 11 and then install MS 13 Platinum? Will installing MS 13 Platinum overwrite MS 11 installation? Or both can be installed and used separately on the same PC?
vkmast is correct. Each install of a major Movie Studio release (not an update to a current release) is done as a separate, independent installation. This is why you can have multiple versions running at once.
However, if you decide to remove the old version and just use the new one you MAY run in to some problems. This is because all Movie Studio version use the same high-level "headers" for both file installations and registry entries.
Just look on your hard drive and see where the different versions are installed. Note that they all start with a folder named Sony. In the AppData\Local folder they all start with Sony\Movie Studio Platinum.
I think you should be pretty safe uninstalling an old version if you use just the standard Control Pane uninstaller. I don't use this because it leaves a big bunch of stuff in the system registry, so I use Uninstaller Pro to uninstall all the software I try out or don't want any more (which is a lot.)
After I upgraded Movie Studio 12 to 13 I ran Uninstaller Pro on MS12 and it uninstalled it fine. However, when it got to the registry entries I had to manually go through the list (which was quite long) and de-select the entries that were used by MS13. This left just the entries for MS12 to get deleted, and they were, with no harm to my MS13.
Another note: MS13 defaults to a different My Documents folder from MS12 for it's project saves. The folder names are "Movie Studio Platinum 13.0 Projects" and "Movie Studio Platinum 12.0 Projects", respectively. Since I had deleted the MS12 program and wanted MS13 to still be able to access all my old MS12 (and previous versions) projects I simple deleted "Movie Studio Platinum 13.0 Projects" and renamed "Movie Studio Platinum 12.0 Projects" to "Movie Studio Platinum 13.0 Projects."
The smart thing to do would probably be to rename this folder to "Movie Studio Projects" and then change the program's settings to use this instead of the default.