If we make the plates look more heavy duty (by adding a second layer of plating on top, or overlapping armor like a knight's shoulder, would that bump it up a class? Also, If we add non-conventional pieces of plating, such as a lateral neck guard, would that also bump it up? I would like to use to proper classification during my WIP.
Bulking them out doesn't change the level, the amount of plates do.
If you look at the first post it quite clearly shows you and lists the basic coverage that qualifies as that level of armour, if you've not met the minimum requirements of the next level up, then your still the one below.
FIRST POST, PLEASE CLICK ME 
I'm planning about the same amount of armour as Jango, with only 1 or 2 additions. Apparently in my own head I am now in a grey area...
Jango with additions , depends on what additions I suppose, if bicep, neck and kidney plates then its heavy as below:
•Head - Helmet
•Neck - Neck armor. ARC-style pauldrons are allowed in addition to collar plates, but not in lieu of.
•Upper Torso - Chest plates and abdomen
•Full shoulder - Heavier style shoulder bell that covers the entire shoulder
•Bicep - Bicep armor or elbow and shoulder armor that end up covering this gap
•Forearm - Gauntlets
•Back - Back plate. Capes are allowed in addition to back plates, but not in lieu of.
•Groin - Cod plate. Loincloths are allowed in addition to cod plate, but not in lieu of.
•Thigh - Standard plate coverage or enclosed
•Knee - Separate knee armor or extended shin armor
•Shin - Standard shin type coverage
•Boot - Some form of boot armor
•Backside - Kama or enclosed thigh armor and butt armor
If you are short on any of the above, you are still medium armour, hope that helps