Going by the AVS thread where someone's taken the lid of his A2 and posted some pictures, it looks like Toshiba are using some different chipsets to give a higher level of integration. This would mean that firmware for A2 wouldn't work for the A1, because it would expect different chips and register settings.
Although we don't know for sure about the innards of the XA2, we can expect that to have changed too (it certainly has a different HDMI chip) - and for the software to be incompatible with the XA1.