Is a head abaya considered better than a loose, modest shoulder abaya, and is transitioning from a head abaya to a shoulder abaya considered a regression from what is better to what is less preferable, taking into account that both fulfill the legally required covering?

There is no objection for a Muslim woman to wear any clothing that fulfills the purpose of the obligation of hijab (definition), by meeting the required conditions. Whatever is more modest for a woman and further removes temptation is better, and the head abaya achieves that.