Again, make sure to read what KaOS is and what it is not, it is NOT Arch based either:
If the distro you used had the replace drop-down option, that is distro specific, the replace option was never added to stable Calamares (3.2 series is now the latest stable), and there is just one release now (alpha 3.3) that has the option added), again, since not tested enough, not added in KaOS for this ISO.
Since KaOS uses it's own Calamares fork, nothing is packaged in /etc for conf files, all are in /usr/share/calamares
For UEFI systems, KaOS has never used grub, again, if you read any about KaOS it has been documented for 10 years why systemd-boot. So that video must use a BIOS install, then grub is in use.