I am also trying to update from P01.A1 to P12.A2 and I am experiencing the same problem.
It is a DX4380 with Windows 7.
Did you figure out how to update the BIOS?
After several failed attempts, I found that there was no path to support UEFI support. The Calvin7 response is accurate. I reverted to Windows 7 using "old" drivers from Gateway.