One of the distinct advantages of Principate Legions over their non-Roman opponents was that they are able to cover their bodies against attacks easily with their large rectangular shields while being able to attack their opponents much easier using their short swords(their gladius).With the late Roman army however,infantrymen used spears,oval shields and much longer swords(the spatha). Were late Roman infantrymen actually disadvantaged compared to their predecessors in close quarter combat or did the changes actually increase their fighting ability in close quarter situations?
NOTE:I fully realize that overall,the late Roman army was actually better than the Principate one in terms of tactics,doctrine and equipment.
First of all let me just point on thing, that sometimes appears on threads like this, the myth, yes myth, that the roman army became defensive after the 3rd century is a lie. I only mention this because I saw too many people claiming that the late roman army was using a defense-in-deep tactic, despite evidence of campaigning across the Rhine until the late 4th century.
Well, first both weapons were, originally, auxiliary weapons. This can be noticed on Trajan's Column were it can be seen auxiliary using oval shields. The spatha had also been used by the auxiliaries, especially by the cavalry and by Galic auxiliary.
But the reason for the change can be, on my opinion, on the need of soldiers.
If you ever held a sword in your life, you will notice that your first instinct will be to use it to slash and not to stab.
Now the gladius, as a small weapon, was not very good for slashing and required several months of training to become used to the weapon and to lose the "habit" of trying to use it as a slashing weapon.
The spatha on the other hand is a slashing weapon. If you hold a sword in fighting position your wrist will be leaning slightly putting the weapon on a seventy-five degree angle (unless is a saber or rapier, because the grip is different), with that angle you will use it to slash some 3 out of 4 times. With the spatha you will mostly slash and it won't require much training to become battle-ready, given that all they needed to learn was discipline and how to form the several battle formations, you could skip much of the sword training.
Now on the scuttum vs oval shield.
One reason was economic.
It's easier and faster to make an oval shield and it also requires less materials.
Another, was the way fighting had become in the 3rd century onward.
The army keep on the offensive but the way they fought change.
The desire now became to kill the enemy at a distance and not at close-quarters. This can be seen by the adoption of the spear, as the main weapon, and they also begun to transport half a dozen Plumbata (javelins).
This happened by two main reasons.
The first and primary was the constant need of soldiers. The Crisis of the Third Century killed many people and with the frontiers always under pressure, the roman army begun to prefer to rely on ambushes to destroy their enemies.
With this we arrive at the second reason. While the late army won most of their battles, roman generals begun to avoid open battle, unless they had a clear advantage in manpower.
With the rise of this type of warfare, it also begun a need for a more light and maneuverable shield and the oval shield had all the required needs.
On the end you can say that the changes happened because of the circumstances.
The need of more and more soldiers made the gladius a hindrance, given that the spatha was easier to use and train.
Likewise the scuttum wan't the best weapon for ambushes and fast marches. The scuttum was a shield designed for open battle and slow advances, were you relied on discipline and organization.
The oval shield was more adapted to the new realities. Many times roman soldiers ended fighting alone and the oval shield offered bigger maneuverability.
If i didn't explain something, or if you fell my explanation was weak on some point, point it out and I will fix it.
Hope this helped you.