Only 5 units remain the same pre and post Marius.
Town watch
Cavalry auxilia
Legionary cavalry
Arcani
Gladiator units
Not counting special units like pigs and dogs or artillery. Also by the time you can build praetorian units you'll have triggered the marian reforms so they don't count either. Another note the first cohort units are bascially the same as the early and regular legionary cohort except for 2 things the morale boosting eagle and size its in the 122 or 242 size range. For a side by side comparison.
Velites=Light auxilia
Hastati= Auxilia spearmen
Princepes=Early legionary cohort
Triarii=Legionary cohort
Equites=Roman cavalry
Roman archers=Archer auxilia