A molecular meanfield model for a rectangular columnar to hexagonal columnar phase transition, as observed in case of the HAT series, is presented here assuming the molecules basically to be biaxial objects. The theory is developed for a rectangular lattice having herring-bone packing of the tilted molecules. Various types of phase diagrams for a homologous series are reproduced for different values of molecular biaxiality. Some of them are very similar to the experimentally observed cases but others, particularly involving biaxial nematic phase, are not yet observed. It is shown that a gradual lowering of biaxiality with the addition of flexible chain segments can lead to the full phase sequence observed in the HAT series.