If you install MSI 3.0 over MSI 2.0, MSI.DLL is updated from version
2.0.2600.1183 to 3.0.3790.2538. However if you install MSI 3.1 over
MSI 2.0 (without first installing MSI 3.0), then MSI.DLL remains at
version 2.0.2600.1183 whereas all the other MSI files (msiexec.exe,
msihnd.dll, msimsg.dll & msisip.dll) all go to 3.1.4000.1823.
Does anyone know if the changes to MSI.dll between version
2.0.2600.1183 and 3.0.3790.2538 are necessary for 3.1? If so, MS needs
to change the 3.1 install to allow for updating directly from 2.0.