Parameters for WiX Major Upgrade
When set to no (the default), products with lower version numbers are blocked from installing when a product with a higher version is installed; the DowngradeErrorMessage attribute must also be specified. When set to yes, any version can be installed over any other version.
The message displayed if users try to install a product with a lower version number when a product with a higher version is installed. Used only when AllowDowngrades is no (the default).
Determines the scheduling of the RemoveExistingProducts standard action, which is when the installed product is removed. The default is "afterInstallValidate" which removes the installed product entirely before installing the upgrade product. It's slowest but gives the most flexibility in changing components and features in the upgrade product.