I know it isn't a feature in the game yet, but how do you think re-training ground units to use new templates should work? I'm assuming at some point you'll be able to take an existing unit and morph it into something else using ground force construction complexes, with time and costs based on how radically you are changing the unit. This is rather straight forward and makes a lot of sense. However, merely upgrading units to use new technology seems like it'll be a bit harder...
If you want to, say, just upgrade your existing units to use the most up-to-date armor and weapons tech you'd have to create and research new versions of every element and then create a whole new template and then retrain everyone. This would get old very quickly with even a small army, let alone a galaxy-spanning one.
Instead, I propose there should just be an option to upgrade a ground unit to use the latest tech using your ground force complexes that consumes time and resources. I know it probably more realistic to have to design a research a new tank even if its using the same armor class and same class of weapons, but ultimately I think its better to just streamline the process for getting a linear upgrade when it comes to ground units. There are simply too many steps involved to expect the player to do things manually.