ThinApp User’s Guide : Configuring Package Parameters : Configuring MSI Files : MSIUpgradeCode

The MSIUpgradeCode parameter specifies a code for the MSI database that facilitates updates. When two packages, such as the version 1.0 package and the version 2.0 package, have the same upgrade code, the MSI installer detects this link, uninstalls the earlier package, and installs the updated package.
The capture process generates a random upgrade code based on the inventory name. To ensure that the MSI database versions have the same upgrade code, keep the same inventory name across versions of the MSI wrapper. For information about the inventory name, see InventoryName.
The parameter works only when the MSIFilename parameter requests the generation of a Windows Installer database.
Do not modify the UpgradeCode value unless the new value is a valid GUID.
The capture process can create an MSI file with D89F1994-A24B-3E11-0C94-7FD1E13AB93F as the upgrade code.