After the fall of Rome, how long did they keep the typical legionnaire armor, weaponry and tactics of popular imagination? I'm having trouble wording this and I apologize, but basically when and how did the Western Roman army transform from the legionary style to a more medieval look?
The classic legionnaire equipped with a gladius, scutum, pila and Lorica Segmentata was long gone before the fall of the Western Empire. By the time of the barbarian invasion standard roman infantry, which was rare, most infantry were barbarians who agreed to fight for Rome in return for land, was equipped with mail and a spear. The mail was cheaper to produce and maintain than the rare plate armour in the heydays of the empire and the spear was more effective against cavalry based forces such as the Huns. Also the decisive arm in battle had changed from the legionnaire to heavy cavalry recruited from the upper classes known as Scholae and also used their own version of the cataphract. So to answer your question by the time of the fall of the Western Roman Empire the legionnaire would not have existed and been different to roman infantry of the time.
Related, but not totally on relevant is this - http://thehistoryofbyzantium.com/2013/10/04/episode-37-the-army-of-the-strategikon/
Episode 37 of the history of Byzantium looking at the army at the time. not quite your time period or transition but might be of interest to you.