So far using KDE Neon with an AMD GPU on the first PCIe and an Nvidia on the second. I have a dual boot Win10/Neon system.
Now due to changing load type (from CUDA to 3D graphics) I need the Nvidia on the first PCIe, AMD will be in the second.
Trying the change Neon latest .5 release didn't boot, with a black screen where normally I find the GRUB menu for the OS choice.
Being not an expert want to ask if uninstalling proprietary Nvidia drivers so to start a "neutral" state will enable me to boot normally, or there are other problems.

Can you boot to BIOS Setup? A decent BIOS will detect which GPU is hooked to your monitor.
If you cant see the BIOS Setup screen nothing can be done except maybe a BIOS update.

Cat22 wrote:Can you boot to BIOS Setup? A decent BIOS will detect which GPU is hooked to your monitor.
If you cant see the BIOS Setup screen nothing can be done except maybe a BIOS update.

I've seen the BIOS, but the problem come after choosing Neon as boot target in GRUB
https://i.imgur.com/Tuh3Fnx.jpg

But, just to be clear, you see stuff on screen at every boot when NVidia is in the first pcie slot - is that correct?
Please describe (be specific) what happens when you boot. do you see the grub menu at all? does it skip grub and boot to an OS? (any os)