Harper86 and Cryptonica helped me out getting a backup of the A3 firmware. thanks for that guys! 
As others already found out as well: The A3 and D3 are very similar. I even found out that the software inside the controller chips on the hashing boards is exactly the same.
Out of curiosity I tried to run the A3 firmware on the D3 and the A3 firmware actually detects the ASIC chips and tries to start mining on the D3 hardware. Looking at the logging it's fine, but it doesn't accept the work as Bitmain did actually changed something on the ASIC chips I guess. I do think they used the D3 as a base and stripped out 10 of x11 algorithms.
For now the conclusion is that you can't convert the D3 to an A3.
It probably does need reprogramming of the ASIC chips on the hashing boards itself.
As others already found out as well: The A3 and D3 are very similar. I even found out that the software inside the controller chips on the hashing boards is exactly the same. Out of curiosity I tried to run the A3 firmware on the D3 and the A3 firmware actually detects the ASIC chips and tries to start mining on the D3 hardware. Looking at the logging it's fine, but it doesn't accept the work as Bitmain did actually changed something on the ASIC chips I guess. I do think they used the D3 as a base and stripped out 10 of x11 algorithms.
For now the conclusion is that you can't convert the D3 to an A3.
It probably does need reprogramming of the ASIC chips on the hashing boards itself.