How do I detect if my MSI is being uninstalled due to an upgrade?

Hello everyone.

I need to identify when my MSI package is being uninstalled because a new product is being installed via the upgrade code. I need to do this in either a basic MSI or an InstallScript MSI.

To be clear, I don't want to know if my MSI package is going to perform an upgrade or not (which I believe would be accomplished via the VerProductCompareVersions() function). I want to work on the other side of things: The package being removed as a result of a Major upgrade.

I don't need the solution to be in the form of an InstallScript function necessarily; I just need a method of detecting this.

Thank you.