How to fake MSI in registry

Hey guys,

We're changing from MSI (InstallAware) to NSIS.
In the InstallAware script, a pre-defined variable NEEDSUPGRADE gets set to true whenever a new/older version of the install is launched while a version is already installed.

The NSIS script now can manage older MSI installs and uninstalls them first. But whenever the MSI installer is launched again (e.g. to downgrade), it won't detect the version installed with NSIS.

Does anybody have experience with this ?

Thanks,
Kristof