Recent experimental work demonstrates a rich transition scenario between patterns of different symmetry (hexagons and squares) close to and beyond threshold in a sodium vapour cell with optical feedback dependent on the magnitude of an external magnetic field. We reproduce the observations in numerical simulations and explain them by calculating the quadratic coupling coefficient between inhomogeneous modes which determines the pattern selection close to threshold. The observed scenario is in accordance with expectations based on general symmetry principles and should occur in many systems, but apparently only in optical systems is it possible to tune the parameters continuously over a sufficiently large area to observe the complete bifurcation scenario.