I was afraid of that, but after messing around with the leveling codes for a few minutes I quickly realized that was probably the verdict. Making multiple units with the same graphic image is a good idea, I was also thinking making the unit going into "training" for a long period of time. Basically the unit morphs into the next level over a long period of time, the upgrade would be a long upgrade and free, or maybe a few resources anyway.
How would the special resource way of leveling up units work, I still don't understand it completely