I've been looking into this but so far I haven't been successful. Without getting into specifics too much I need to allow a downgrade. I've read one solution to run a VBscript first then the installer. This option is not possible. I wrote a Installscript custom action that should execute before anything else and if it finds any version of the product it will uninstall it. Then it should proceed as a first time install. However the installer is still being prevented from downgrading. The Custom action is sitting in the Install UI Sequence "After ISVerifySCripting Runtime" and in the Exec Sequence "First action". I even changed the ISPreventDowngrade action to hopefully stop it from preventing the install but with no luck. This is getting installed on a tablet and the user won't have access to the traditional desktop or "Program & Features" so they won't be able to do much. I want my installer to detect any version of the product and if found uninstall it.
My other idea is to automate the generation of the product code and then pass that into the installer. Have the installer put the product code it was built with to a specific registry entry and if the currently running installer sees it then uninstall that particular version.
