Working in an enterprise system. We need to determine if everyone needs to update to 10.6 at the same time for mxd cross-compatibility or can we update from 10.5 as needed.
You need to save a copy of the mxd to an earlier version if you have saved the mxd in ArcGIS latest version 10.6.
Based on ArcGIS help: Saving a map:
New documents cannot be opened in earlier versions of the software; however, you can use the Save A Copy command to make a copy of a map document so you can open and work with it in previous versions of ArcGIS.
As @JordanMiller mentioned, the MXD Doctor can fix the mxds, but not just looking backwards, but from future releases. I just used the 10.5 MXD Doctor to backward save some 10.6 mxds for use in 10.5 (I had to downgrade for HEC-GeoHMS and HEC-GeoRAS, and didn't think to save as before uninstalling 10.6).
I have had a similar issue and I used Python script to fix it... You need to open a blank document in ArcMap, then open the Python window. Then copy/paste the following code, just repoint the path to the proper mxds...
mxd_path=r"path/to/the/10_5_mxd.mxd" mxd= arcpy.mapping.MapDocument(mxd_path) mxd.saveACopy(r"path/to/the/new/10_4_mxd.mxd","10.4")