Windows Small Business Server 2003 and 2008 (SBS2003 and SBS2008) are still widely in use, but CALs for these are no longer available for purchase. So, what if you require more CALs but cannot get rid of your old SBS?
SBS 2008 and SBS 2011 work on a trust-based CAL count, so they do not need activation. You can simply purchase SBS 2011 Standard CALs and exercise your downgrade rights. Note that this cannot be done with SBS 2011 Premium CALs as they contain features that are not the same as in SBS2008 Premium.
Unfortunately, for SBS 2003, it's not that simple, as it uses online activation for CALs. In this case, you still have to buy SBS 2011 CALs, which are legally usable on your SBS 2003, but in order to circumvent the issue with online activation, it depends through which channel you can purchase these CALs.
If you can purchase SBS 2011 CALs through Volume Licensing, you can use the VLSC to downgrade your SBS 2011 CALs to SBS 2003 (R2), and use these keys to perform an online activation on your SBS 2003.
For channels other than VL (FPP, OEM, ...), there's no way to downgrade the SBS 2011 CALs. In this case, you can use the activation keys that were distributed by Microsoft to perform an online activation:
|20 CAL keys||5 CAL keys|
Before adding these keys, calculate the actual number of CALs you need, and go from there. I suggest you start by using the 20 CAL keys first, then use the 5 CAL keys to get to the CAL count you need. If you do it the other way around, you risk hitting the 75 user CAL limit (whic is the maximum number of CALs for any SBS), which will fail the addition of that particular CAL key.