Comment 4 for bug 1879466

Note that while switching the metapackage is not a typical operation on a regular installed system---PCs tend to be either BIOS or UEFI without switching from one to the other---it does come up when you are installing a standard OS image that has the "wrong" GRUB type installed and needs to change over to the other one.

Beyond the issue with /etc/default/grub, the ideal solution would be to allow both grub-pc and grub-efi-amd64 to be installed simultaneously. Not only would that eliminate the need to switch metapackages at all, it would allow for installing a dual-mode BIOS+UEFI bootloader setup (e.g. for a Linux system running off a portable USB hard drive). See https://bugs.debian.org/904062