Software Deployment

msiexec /x filename.msi /qn

• You can locate the required code to pass to msiexec.exe /x by opending regedit.exe at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall and search for the application name (or just browse through each sub folder till u find it).
• When you have found it you can pass it to msiexec as explained above: msiexec.exe /x {0077A7C7-3333-2222-1111-111111111000} /qn

You can add an upgrade code for version 7 in your version 8 package if you have to do it through an msi.

RemoveExistingProducts Action should be moved just before InstallInitialize.

I would suggest you to uninstall it using {ProductCode} as SMal.tmcc mentioned in his post rather than uninstalling it using the source msi.