Under normal circumstances upgrading your OS does not pose any special licensing issues. However, certain hardware changes have occurred since licensing can combine with the OS upgrade to trigger the license software to view your game as potentially pirated. Basically, it now thinks it has been moved to a different computer. Fortunately, this issue is easily cleared up.
For games that use eLicense (most of the Strategic Command titles, all the Theater of War series, and a couple of the older Combat Mission games) the solution is often as simple as reentering the license code. However, if the two activations for the license have been used then licensing will fail. If that happens the solution is to file a ticket with the Help Desk. Make sure to provide the name of the game/s and your license/s for the affected product/s.
For games that use Gameshield (some Strategic Command titles and most of the Combat Mission series) the process is very similar to that of eLicense. In the event that relicensing doesn't work file a ticket with the Help Desk that includes the name of the game/s and license/s that are affected. You will receive small EXEs that are specific to the game in question. The EXEs reset your license completely so that you can relicense with a clean slate.
If you do not know what your license number is you can find it by logging into your store account and viewing your order history. For products that were not downloaded the license code is found on the back of its CD/DVD case.