Both hardware versions use the same firmware, so the changes shouldn't impact the usability of the DNS-325 from that perspective. In other cases, such as the DNS-320, a "B1" hardware version was released that uses a different firmware family as the "A" series.
As far as differences between hardware versions are concerned, newer hardware series typically have subtle differences in engineering specifications to improve upon the design (i.e. slightly different components, different power supply etc.). I can not speak to the differences specific to the DNS-325 "A1" and "A2". Requesting a replacement "A2" model or sticking with your "A1" model is a judgement call that you'll have to make.