There is always a chance of bricking the GPU when you flash the BIOS. It can happen unexpectedly. I've flashed maybe 30 GPU BIOSs, and 3 of them bricked. However, there is still a way to un-brick them. So I haven't actually lost a GPU doing it because I was able to force a flash using the command prompt. You have to have a backup GPU though to get in your computer.
Why do you want RX 470 on an RX 570? Typically its the other way around.