When the first Hakkas moved to Fujian, they were the imperial court officials. They could afford to build the very extravagant houses  五鳳樓 (Phoenix house) that were modeled against the imperial court. Unless the inhabitants were authorized by the emperor, it is unlikely that they could build the houses in this design with such  decor, without risking their lives for violation of the imperial court.
　

In the mid stage, the Hakkas were losing their support and affiliation with the imperial court. They had to compete with the locals on equal (actually less equal) grounds. Failed to have the blessings and protection of the emperor (who was in trouble himself), they were often attacked by the locals. The round houses土樓，圓屋 were built as a very defensive structure to fend off the locals.
　

In the final stage of migration, the Hakkas were more or less integrated with the locals. There was less need for protection and defense. To befriend with the locals, the 平房  were built as a symbol of openness. Piang Fongs also could appear early as the dwellings for the lower class (servants, lower rank officials) who accompanied the nobilities in the southward migration.
　

There is some indication as we learn that the phoenix houses were abundant in early Hakka settlements in Fujian. The Round houses were common in upper(eastern) Pearl River of Guangdong, while near the Pearl delta, mostly we see Piang Fong. In my home village in Dongguan,  I have not seen Phoenix house nor Round house.