You can only change it if it exists. If you want to install your app somewhere else, and the INSTALLDIR property 'is not working' then the MSI might not be using INSTALLDIR. (check the Directory table for INSTALLLOCATION, APPDIR, also, others are available)
I have also seen with a few vendor MSI's that a CustomAction sets the install location (INSTALLDIR or other) , so you set it to what you want, and then the CA overwrites your change. grrrr
OR, the CA only runs during InstallUI sequence, so you install it silently and the app installs to the root of the C:\. double grrrr.
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