Due to a little boondoggle, some users may have trouble upgrading mesa because pacman can't figure out the dependency tree. If this is the case, and pacman tells you that it can't satisfy dependencies on mesa-libgl, then you should be able to resolve this by manually telling pacman that the resolution is libglvnd:
# pacman -Su libglvnd
Additionally, multilib users may need to manually downgrade from the no-longer-existing libre-multilib/lib32-mesa and friends to multilib/lib32-mesa and friends.