Recommended Practice

ElumTools is not backward compatible between minor versions.  This means that once a Revit file has been opened on a machine with a given version of ElumTools, that file can no longer be used with earlier versions of ElumTools.  For this reason, it is highly recommended that all users on a given project have the same (ideally latest), minor version of ElumTools installed for the applicable Revit generation.

Downgrade procedure (use with extreme caution!)

As a contingency, ElumTools provides a method to downgrade projects when needed.  This should only be used when going between the listed minor versions, or with the guidance of Lighting Analysts staff.  Otherwise, data loss and or corruption may occur.  Lighting Analysts does not accept responsibility for problems with files that have been downgraded under any circumstances!

Files saved in these versions...    
Can be downgraded to these versions safely
2018.6, 2018.7, 2018.8
2017.9, 2017.10, 2017.11
2016.13, 2016.14, 2016.15

To downgrade...

1.    Open the version of Revit with the legacy version of ElumTools installed.  
2.    Check the Allow Downgrade box in the ElumTools settings.
3.    Open the file of interest and choose the downgrade option, when prompted.  ElumTools will then be operational.