When you buy a license to run an operating system such as Windows 2000 Professional you can't use that license to run Windows XP on some other machine when your Windows 2000 Pro system is not running. However, the opposite action is allowed. You can apply your Windows XP license to a system with an older operating system such as Windows 2000 when your Windows XP license isn't being used. Each vendor has a somewhat different practice, but if you can prove that the number of copies of software in use doesn't exceed the number of current licenses you generally are operating acceptably. Downgrading is something that many vendors will support, although here I use Microsoft as an example.
License pooling is of course common, and its standard practice to buy a number of connection licenses for a server or database. That license if for a pool of connections, and it doesn't matter what versions of the OS are being connected. Downgrading is however different. Here you have to account for what exactly is connecting, and older systems can only replace newer ones. For license compliance you need to be able to prove that at any one time you have the requisite number of licenses and types of licenses to satisfy the software vendor's EULA (End User License Agreement).
For example, Microsoft allows downgrading on a case by case basis. If you implement a solution that measures licenses you may find that you don't need to write a permission letter to Microsoft in order to use downgrading. It may already be part of your management package.
Why would you want to downgrade? Say for instance you have an application that ran on MS-DOS. It didn't need a GUI interface and it required some very specific drivers found in that operating system. This was in the bad old days when an operating system was meant to be lean and mean, RAM was at a premium, and developers' code was optimized to run small and fast. That OS might be running a CAD output device and your shop still gets useful work out of it and you need another system just like it. You can't buy a new copy of DOS (although there's always eBay), but you can buy a copy of Windows XP and apply that license to a system running DOS. You might want to check to see if there is any inventory of older OS versions from Microsoft by calling them at (800)426-9400. Chances are (but not always) that the inventory will be cheaper than downgrading.
Later versions of the Windows operating system may already have language written into them to allow this practice. There are some provisos here, and plenty of gotchas. For one thing you can't downgrade a version of Windows that is an OEM version preloaded on your system at the time of manufacturing. Microsoft didn't get a full payment for that version of the software. Microsoft Select customers must go through their vendor to get this permission. You may also find that vendors who sell Microsoft software either separately or on hardware have the ability to sell you an older version of the software as well.
What about the situation of donating software products. For the license to be honored you need to file a License Transfer form with Microsoft. Other vendors will simply accept some form of a proof of purchase, which is the first page of the manual, UPC on the box, or your first born child. It all depends on how hungry that vendor is to get you into the upgrade cycle or make sure that they get that first sale. For charity and especially for donations to a not-for-profit institution the rules are much less restrictive. You can see how to transfer licenses with Microsoft products at www.microsoft.com/giving/.
Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.