The first Roman reform is the Polybian one, which is not that noticable. You can recruit most of the old Camillian units under the same name, they are just better equiped now. This is especially noticable with Triarii (no longer a phalanx, just an ordinary spearunit) and Principes (donned armour and traded spears for swords). Also, Rorarii, Leves and Ascencii are replaced by Velites. If you can still recruit these old skirmishers in your cities, you are experiencing a bug. Unfortunatly, already recruited units will not be upgraded to their Polybian equivalents, so existing Rorarii will not be changed into Velites.