CAD tip # 6476:

One of the advantages of the Autodesk Subscription program is the right to use older versions (up to 3 versions older release) of the CAD application you own (e.g. if I own AutoCAD Mechanical 2009, I can also use Mechanical 2006). This applies even when you have never owned that particular older version. If you do not have it, you can download it from your Subscription Center and claim a separate serial number from Autodesk.

Such older versions cannot be used to increase the total number of the licenses you own - you cannot use them concurrently to your current license (e.g. for another user or computer). This right to use older versions is in effect only for the period of validity of your Subscription contract (renewable).