This post may be somewhat on the late side, but given the continuing confusion in the Discussion Groups, it may still be of value to some. The 2010 release of AutoCAD® and the related "vertical" products, such as AutoCAD® Architecture, introduced a new AutoCAD file format (the 2010 format). ACA files created in 2010 as well as files created in a previous release and opened and saved in 2010 will have 2010-format AEC objects, which are NOT compatible with previous releases and CANNOT be saved back to a previous release format.
As this Knowledge Base article states, there are only two options for going back to a previous release. Exporting to AutoCAD will explode all AEC objects to AutoCAD objects. SaveAs to a previous release will result in un-editable proxy objects representing the AEC objects and new AEC objects will NOT be able to be created in the file.
So, if you need editable AEC objects, there is no going back from a 2010-format file to an earlier release. Keep that in mind when planning a deployment of ACA 2010. And, for the record, this is exactly the same situation as occurred with the release of ADT 2004 and ADT 2007.