When I ran 'pacman -Syu' today on an rpi3 Arch-Linux installation, the following question was asked: Replace linux-raspberrypi with core/linux-rpi-legacy? [Y/n] Is this a simple renaming or is this an important change impacting the system's future?
Always look first into the forums: https://archlinuxarm.org/forum/viewtopic.php?f=3&t=15683