Cannot change project file name during project upgrade

We have a custom project type that we want to upgrade. We implemented all methods of IVsProjectUpgradeViaFactory on our project factory. During the upgrade process, we need to rename the project file name because the extension has changed.

IVsProjectUpgradeViaFactory.UpgradeProject has an out parameter "pbstrUpgradedFullyQualifiedFileName" described as "Copy upgrade will have a different path - others may change the project name.", which we assume can be used to specify a new file name for the upgraded project. 
However, this value is not picked up by the project factory and we end up with the same file name at the end of the upgrade.

Are we missing an interface or did we assume the wrong meaning for pbstrUpgradedFullyQualifiedFileName?