If a Roman soldier from the 2nd century were to awake one morning and suddenly find himself serving during the reign of Diocletian or Constantine, what major differences would he immediately notice?Which aspects of his military service would change the most?

by Cato_the_Cognizant

Beyond differences in equipment, would he still recognize the basic practices, tactics, and structure of the army? Or would it mostly all seem new to him?

From his perspective, would the overall training and discipline of the army appeared to have changed much, and if so, to what extent?

gingersaurus82

Here's an old post that somewhat covers what your asking, with /u/celebreth discussing the armies of the Principate era, and linking to another answer by /u/Ambarenya about the Dominate era armies, which should be able to give you somewhere to start, if not answer your question directly.

https://www.reddit.com/r/AskHistorians/comments/1pch88/how_did_roman_military_tactics_and_army

Edit to add a direct link to the answer by /u/Ambarenya here https://www.reddit.com/r/AskHistorians/comments/1n8yrn/the_late_roman_military/ccgj6dm