Romans regarded Julius Caesar as the first emperor , as I understand it, whereas modern historians usually give that title to Augustus. Why the change?

Ok, Barnes is making the claim that in Late Antiquity Caesar was viewed as the first emperor by "the majority of both writers and the ordinary population.... " How he knows what the ordinary population thought he doesn't say (I have the impression from some of his other works that he often has confidential information about other people that he doesn't share with his readers).He seems to be conflating being a monarch with being emperor, and I'm not sure that's fair. We'll come back here in a second. He provides as evidence the inclusion of Caesar in collections of lives of emperors like Plutarch's mostly lost set of lives, and Suetonius', as well as later chronicles and speeches.

It's important to note that Barnes is not saying all Romans in Late Antiquity held the view that Caesar was the first emperor. He explicitly says there was no consensus on the matter. He mentions several LA writers that consider Augustus the first emperor, such as Melito, a Christian writer who linked the monarchy of Augustus to the spread is Christianity.

When Barnes introduces writers or passages that treat Caesar as the first emperor, he emphasises the role Caesar plays as a marker of time indicating a transition from Republican rule to monarchy. Some of these chronographical writers get the facts quite jumbled, like claiming Caesar's mother died in childbirth and that Caesar killed the entire Roman Senate.

So that's what Barnes is talking about. It is not the case that the opinion that Augustus was the first emperor is a modern opinion; it is ancient. So the question of a "change" is moot. Why modern scholars prefer Augustus I can guess at: most of us don't read these late antique chronicles. According to Barnes they say little about Caesar except as a temporal marker and as I mentioned some of them get things quite wrong. People studying Caesar are more likely to look at fuller sources and sources nearer to Caesar.

Now about the conflation of monarch and emperor, there are different sorts of rulers. Being a Roman dictator is one sort of thing and a tyrant of the Greek kind something else. Barnes seems to be taking any reference to Caesar as sole ruler as equivalent to being "emperor." For instance the passage in Josephus, BJ 1.11.1, describes Caesar "having possession of the state" ("kataschonta ten archen" in Greek), and that is not the same as the quasi-constitutional position of Princeps, which we have come to call "emperor." I feel like this is a little sleight of hand on Barnes' part to gather evidence for his claim.

EDIT: goddamn typing on my phone is bad.