Installing macOS High Sierra on Mac Pro 2009 with a firmware upgrade requires a firmware update - is it safe? I have a 2009 Mac Pro, which has been upgraded to a MacPro5,1. It's currently running macOS Sierra. I was going to upgrade it High Sierra, but the High Sierra installer said it needs to first update the firmware and asked me to proceed in doing that.

I'm nervous to allow this to happen since the firmware was already upgraded from a MacPro4,1 to a MacPro5,1.

Also, I'm running a flash NVIDIA GTX 980, which has the boot support. I'm afraid this might cause problems as well.

Are there any known issues or conflicts with my configuration or similar configuration when upgrading to High Sierra?
I was able to perform the firmware upgrade with no problems. The MacPro upgraded the firmware after I held the power button until it flashed, then prompted me to install High Sierra after. It rebooted a few times in-between.