Hello Everyone !
Hope you’re fine ?
Considering to replace my GPU (Actually Nvidia RTX 2070) to an AMD RX 7800 or 7900, I would like to know if it’s better to reinstall the whole system because there’s a lot of files for nvidia (kernel, grub, and many others…) Or is there a easy way to switch this without reinstalling Endeavour ?
It’s not for tomorrow so I’m not in a hurry, but I prefer to take informations before 
Many thanks to all !
No need.
Just remove nvidia-specific packages and configs.
If you havent made any configurations then that would be nothing.
Packages you could search with something like
pacman -Qs nvidia
Aside from removing nvidia packages you may wish to add some amd packages;
sudo pacman -Syu amd-ucode lib32-vulkan-radeon linux-firmware-amdgpu vulkan-radeon xf86-video-amdgpu
Then just rebuild initram and refresh your bootloader.
If grub (and dracut) is correct then
sudo dracut-rebuild
sudo grub-mkconfig -o /boot/grub/grub.cfg
Remember to check that either of these are supported by the current drivers so you don’t have nasty surprise. There can be hiccups with AMD too. 
Yeah ok 
I will try.
And last question :
Can I use another AMD card (RX550) temporary ? Because I need first to sell my actual Nvidia, then find a new AMD in second hand (not enough money for a brand new one…)
Is this the same drivers for all AMD cards ?
Many thanks
Yes you can!
Sure. I am using a newer RX590 but still older compared to the current cards. It works fantastic.