I used dnf to system-upgrade to F40, which downloaded all the relevant packages, and then I rebooted through dnf. The restart updated all the packages and automatically rebooted when it was finished. When the grub menu was displayed after the reboot it was obvious that the upgrade did not update the grub config as there was no entry to boot off the F40 installed kernel. I booted to the display manager which I think is still gdm, and the first thing I noticed there was the "plasma on xorg" selection had been removed but the "Gnome on xorg" and "Gnome Classic on xorg" were still there. I selected the "Plasma" entry and booted into Plasma. Having loaded Plasma I then went into the system entries menu and went through all the options again, and it was obvious from this that the display options I had configured with F39 had been wiped with F40. I configured the display settings again and set the new HDR option as I have a HDR monitor. After the configuration changes I rebuilt the grub menus using grub2-mkconfig to get a boot entry for the new kernel, and rebooted. With the reboot from the new kernel, and for that matter any of the older kernels, I got a message that the nvidia driver was not found and it was falling back to the nouveau driver, I don't know how as the nvidia driver was black listed in the grub menus. From the display manager I loaded Plasma as I did after the first upgrade boot, and Plasma displayed a black screen and never went any further, irrespective of how long I left it for, and the only way I could get out of that state was to use the physical reset button on the computer. On reboot, if I selected "Gnome" or "Gnome on Xorg", gnome would start up quite happily, but logging out and starting Plasma would still hang the computer. So rebooting again to the display manager login screen, I used ctrl+alt+F2 to switch to a terminal login process. Logging into the terminal the first thing I did was use mokutil to check the uefi status and it told me that uefi was disabled even though it was enabled in the bios. I checked whether the signing key was enrolled and it was, but the system wasn't using the nvidia driver even though it was installed, and a reinstall of kmod-nvidia and akmod-nvidia did nothing to alleviate the issue. So I uninstalled the kmod-nvidia module, and did a force re-enroll of the uefi signed key (potentially with a new key), and then rebooted to go through the mokutil enrolment required at boot. This did not resolve the Plasm start issue, so I loaded Gnome. Once in Gnome I started Firefox Nightly to do some net searches to see if I could find a solution, and the one thing that did happen at this point was the boot did not display the "falling back to nouveau" message, but the start of firefox displayed a message that a gpu couldn't be found on pci. Looking for a resolution to this I found an entry about, as part of installed nvidia drivers, to ensure the gpu firmware was also installed. So I did a dnf install nvidia-gpu-firmware which was then installed as it hadn't been already. After installing the firmware I rebooted and started Plasma, which successfully started without issues, and when I checked the video driver it was finally using the nvidia driver. This was a lot of work to resolve, and is the worst experience I've had with any fedora system upgrade.
regards, Steve