I have created in the past an MSI which I have used to deploy the application.
Now, I want to distribute a new update of the product with an MST together with the MSI (since I have changed the MSI and I want some transformation to be applied). My problem is that this transform file is only applied on new installations, but it seems that it is not applied on already installed places.
In order to fix this, the only way I have found is going to HKLM\Software\Classes\Installer\<GID> and add the MST there in the Transform key. But that requires a lot of manual work, like finding what is the actual ProductCode to add the key to, copying the mst somewhere permanent, etc. Can you think on any other way to get this done?
What's the update to the product? If you build a major upgrade then just apply your new transform when you install the new upgrade MSI file.
You shouldn't really expect to successfully hack the system enough to think there's a transform present that really isn't! It would be far easier to change one file in your product and create a patch that will include all the changes in your transform together with one new file. A patch isn't just a new file - it's also the changes between the two MSI files, and doing that is safer and more reliable.