Vertical AutoCAD programs like Architecture and MEP are not backwards compatible like vanilla AutoCAD is. It is a very big misconception throughout the industry and we (us techies at MasterGraphics) are trying to educate folks on this before they ruin some of their production drawings. I have run into this many times in the past and have been trying to spread the word.
AutoCAD Architecture (or ADT) is not backwards compatible with older versions of ADT. When you perform a save as, only the AutoCAD objects get saved back, not the AEC (architectural) objects. Yes, you can open up the file in the version you saved back to, but you (or the person you sent the file to) will get a bunch of ARX errors and will have nothing but trouble. You can perform an Export to Autocad, but it breaks all of the AEC objects down to regular FLAT AutoCAD objects.
Many also think that Object Enablers will allow them to work with older or newer objects, which is incorrect. Object Enablers only work for programs of the same version of different verticals such as MEP, Civil or plain AutoCAD.
I run into this problem every year when the new versions come out as well...
If you install a trial of AutoCAD Architecture 2008 and opened and saved a 2007 ADT drawing, users of ADT 2007 will not be able to open the drawing without getting ARX errors. In order not to get the ARX errors in ADT 2007, the drawing will need to be broken by doing an Export to AutoCAD. By doing this, it basically makes the file "useless" to AutoCAD Arch since you no longer have AEC objects. Luckly when you export to AutoCAD, by default it will give an ACAD prefix to the file it's saving; so it creates a new file. Unless of course the person overwrites the existing file, then you better hope you have a backup! You can check the setting for the prefix (and even change it and other settings) by going to Format->Options->AEC Editor tab.
Yes, you can save back to an older version and view it, but you won't be able to work with it.
