Surely this is a problem of training rather than one of different shaped tins? Nearly every POL product that comes in 5 litre quantities comes in the same 5 litre container yet the only regular mix up you hear of is that of OM15/OX26.
When I worked in POL I would never assume that just because a tanker that had arrived to deliver fuel had Avtur markings on it and all the paperwork agreed that the product inside was Avtur that it was. I and my colleagues QA'd every single fuel load that arrived because not only did we have professionalism in the task in hand, no matter how monotonous it may be, but we fully understood the logistical and legal repercussions if we messed up.
BTW, the reason for the change of the containers is cost but the cost cutting is ultimately down to the container manufacturers themselves. The containers are a standard UN approved type and although the old round ones are still acceptable under UN regulations the civil sector won't use them as they take up more room during shipping (and hence cost more to move) than the rectangular ones. As a result of these market forces it would be incredibly cost prohibitive (more than the 40p per can quoted earlier I believe) to produce them just for military usage.
Perhaps a reusable round shaped shroud to fit over in-use cans could be fabricated locally in-lieu of training folk to read the label. You can use that for a GEMS suggestion for free if you like.