SBS 2011 Essentials Trial and Registration

We have installed SBS 2011 Essentials on a Dell PowerEdge server as a trial.  We're now at the end of the trial and would like to purchase the correct SKU to activate.  However, I'm finding that Microsoft SBS 2011 Essentials is hard to locate and is usually out of stock.  If we purchase SBS 2012 Essentials, can we use that keycode to activate SBS 2011 Essentials?

Thanks.

Ken