Saving to previous versions of ArcGIS in InfoSWMM and InfoSewer
Once you open and save an existing map document (.mxd file) using ArcGIS 10, the map can no longer be opened with earlier versions of ArcGIS because it will now reflect the new functionality added at 10. Similarly, new documents you create with 10 also 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. With ArcGIS 10, you can save to ArcGIS 9.3, 9.2, 9.0/9.1, or 8.3. ArcGIS 9.0 and 9.1 map documents are directly compatible with each other and those versions of the software.
Each new version of ArcGIS introduces functionality and properties that aren't available in previous versions. When you save a map document, layer file, or 3D document to a previous version of ArcGIS, the format of the file is changed to eliminate properties not available in the older version.
This means saving from 10 to a previous version removes from the file any functionality that depends on the newer software in ArcGIS 10. Therefore, some work may be lost if you save to 9.3, 9.2, 9.0/9.1, or 8.3 and start working with the older copy again in 10, since the 10 functionality was stripped out in the Save A Copy process. Your original ArcGIS 10 file will still have the new functionality. Source http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//006600000253000000.htm