Buying ATIH 2012 after free trial

I downloaded the free trial version of ATIH 2012 (which I believe is the full version) and now want to buy it. However, I can get it from Amazon cheaper than Acronis themselves are charging. So, if I buy a boxed version from Amazon, can I just type in the serial number that comes with it into the free trial version that is installed on my computer? That is, not actually installing the boxed version, but just using its serial number. (I can do this quite legitimately with other products, such as Norton Internet Security, that I can purchase cheaper than from the original supplier).