O wners of a modern UEFI system should (as long as you can get into the UEFI settings) be able to revert any unwanted changes caused by installing a new operating system. The UEFI boots an image from the EFI partition or live media, and userspace tools (both on Windows and through
efimgr
on Linux) can change the boot order and in some cases break things. It’s generally expected (but not necessarily desired) that a newly installed OS will put itself at the top of the boot ordering. Often this results in Windows booting immediately and the only way to get to the UEFI settings to revert this is to hold the Shift key while shutting down from the start menu. An option to reboot to settings should appear.
Our experience was, mercifully pain-free though. We upgraded Windows 10 on a 10th generation Dell XPS set up to boot Ubuntu, and lo and behold the default boot choice (GRUB) was left intact. Not only did Ubuntu boot correctly, but the old GRUB entry for chainloading Windows also worked. We think we got lucky here, but even if we didn’t, booting Windows would just be a matter of entering the UEFI settings or summoning a boot menu at startup. If UEFI didn’t give us these options, then we can reboot to the firmware setup with