Googling this indicates that the 4.0 -> 4.2 update is partly firmware and partly hardware, so there has been a hardware change. AFAIK, there's no date available for the change and no way to ping the hardware to determine the BT version. If your only use of BT with the iPad to connect keyboards and headsets (which is all I use BT for with an iPad), the change is basically a non-event, as far as I can tell from searching for info on the change.