Changes in the Roman Army

[Nathan Ross]

the establishment of a field army with the vexillations and cavalry vexillations grouped in Macedonia..

Evidence for Gallienus's 'battle cavalry' is actually a lot shakier than it might seem. Duncan Campbell's article in AW II.6 suggests that the famous 'Dalmatian cavalry' was actually the Equites Singulares Augusti, not a new elite formation of specialist field cavalry detachments. A good example of the cloudiness of this whole era!

Nevertheless, Gallienus's unusually long reign by 3rd-century standards would provide the best opportunity for a revision of the army, if such a thing took place - it depends whether you believe that the army inherited by Diocletian was substantially different in structure to that of Severus, or not.