As far as I know you should be able to run the QV9 plugin silently right off the bat and it should upgrade the existing 8. Otherwise look into doing a silent uninstall first.
Here's an example for installing the QV9 Plugin:
msiexec /i C:\Folder\QvPluginSetup.msi /l*v C:\Folder\Installation_Log.txt /quiet
If you just type msiexec in a command prompt or Start > Run, you'll get information on the different switches that are used in msi packages.