Didn't you read what I wrote? I haven't said it's not software compatible to older cards, but hardware based a R9 2XX series will not support it 'fully'...
The existing AMD cards will probably not have the hardware to fully support DX12. If thats the case they will support DX11.3, which is basically a slightly inferior version of DX12 that will run on older hardware.
Nvidia's Maxwell (GTX 970, 980) are the first cards to have official full hardware support for all features of DX12. The older Nvidia cards will probably join the 11.3 camp with AMD.
But DX12 is not out yet. It will probably take over two years before DX12 titles come out.