Haha, I'm also trying to create a historical late Roman army.
As for why they changed their equipment. I've thought about that, and you have to take into context the situation the Romans were in. They were fighting not only wars against foreign enemies but themselves too. The death toll is high. This creates a lot of empty spots in the army, and as Rome needed soldiers and needed them NOW they had to lower the standards a little in training or discipline. They also had to let go of fancy and pretty equipment and use only the practical. As they conscripted quickly they needed weapons and armor RIGHT NOW! They simplified their arms and armor so it would be easy to produce quickly. Their enemies were numerous, from horse riding archers to melee infantry the legionary needed to be able to respond to all threats, thus the changes in equipment so they might respond to any situation.
They needed to march to battle anywhere as quickly as possible, which is why their equipment became lighter. It also helps that they wont be as fatigued and are better able to chase raiding bands/armies of German and Persian scum
Romans in this period have not the raw aggression and glory seeking their forefathers had when creating the empire, they just wanted to keep what they had. It would have helped if they didn't fight so many civil wars. The empire could have lasted far longer!
@Therazaman
I have the same book, great stuff