Is it possible to downgrade the data in our 3D DGN models to an older version of OpenBuildings Designer, for example, going from Update 8 to Update 6?
While much depends on the elements that were placed in your models, unfortunately there is no process for downgrading native OpenBuildngs Designer elements created from one version to accommodate the requirements of an older version. Upgrading to a new version is certainly possible, but that is because the new version "understands" what previous versions included, while those previous versions cannot interpret new schemas and data that the new version may have created. This is why all new installations display a "...is not backwards compatible" message on startup as discussed in this Forum thread.
And while you can simply open files created in Update 8 within Update 6 and continue working, there's a good chance that something will not work as expected along the way. This happens most often due to DataGroup schema changes from version to version. While this case outlined in this wiki is a bit more extreme as it mentions going back and forth between CONNECT Edition and V8i, the same concept can apply to all versions.
This wiki outlines where to find additional information about changes made from version to version.