The transition from four legs to two legs could be considered both progression as well as regression. If you look at it from the perspective of the pigs, then it absolutely is progress. It is an important stage in their transformation (specifically Napoleon) into the similar figure of a human-like dictator that once tyrannically, callously, and with self-indulgence, governed the farm (allegorically connected to Stalin’s evolution into a tyrannical leader after the overthrow of the tsar, Nicholas II of Russia). Orwell does an excellent job demonstrating this correlation when he creates a revelation with the “other” animal characters that Napoleon (and the other pigs) is now discernable from the other humans in the room. By the end of Orwell’s novella, the pigs had effectively moved into the role of a self-serving government whose “progress” on the farm was founded upon not only the exploitation of the lesser, and significantly less intelligent, animals, but also on the use of manipulation tactics aimed at keeping the exploited animals subservient through menial labor, lack of rights and the removal of their voices.
However, if you consider this question from the alternate angle of the farm animals, then it is a regressive action. The non-pig animals begin with a hope and dream of utopia through socialism only to have it destroyed by the manipulative and malevolent methods of self-serving “comrades.” For example, due to their lack of intelligence—evident in their continuous inability to recognize the changing rules, the shift in labor and the special allowances being extended to only the pigs— they remain in a subservient role until it is too late. This allows Napoleon’s transition to take place, along with the other pigs. Many of them don’t even recognize the atrocity of Boxer’s murder as an indicator; in contrast, most of them continue to follow the glittering rhetoric provided by Snowball as he contorts the truth to comfort them. So, the pig’s transition from four legs to two legs could actually be considered symbolic of regression as it relates to the destruction of the socialistic ideal initially glorified by Old Major at the beginning. In addition, the transformation from four legs to two legs also substantiates Orwell’s theme of corrupt governmental institutions finding success through the disenfranchisement of its ‘people.’