Consider the implications of upgrading projects from or to certain versions of Revit.
When a user loads a Revit 2012 version of an .adsklib file in Revit 2014 or later version, the file is copied to the current location, and the file name is incremented. For example, in Revit 2014, test.adsklib displays in the Material Browser as test(1). In the file system, the file displays as test(1).adsklib, leaving the original file intact.
When a user loads a Revit 2012 version of an .adstlib file, the file is copied to the current location, and the file is assigned an .adsklib file extension. For example, in Revit 2014, test.adstlib displays in the Material Browser as test. In the file system, the file displays as test.adsklib, leaving the original file intact.
In Revit 2012, libraries contain property sets and do not contain any materials. If you use the Material Browser to load a Revit 2012 .adsklib or .adstlib file, materials will not display, and therefore a warning symbol will display next to the library name. The data in Revit 2012 property sets will be migrated as assets in Revit 2014. In order to access Revit 2012 property sets as Revit 2014 assets, you must use the Asset Browser to load the Revit 2012 libraries.
When a user loads a Revit 2013 version of an .adsklib file in Revit 2014, that library file should open in Revit 2014 with no problems.
When you upgrade to a new release of Revit, material structural properties are also upgraded. Pre-2013 Revit materials contain structural properties labeled as the Structural properties set. In Revit 2013, these properties were migrated to the Physical asset.