I stumbled upon a method to not only replace obsolete instances of models with newer ones at virtually no cost, but to also repeatedly generate free resources and population. I've tested this exploit over 7 times in a row now for 8750 free metal and 28,600 free population. Theoretically I can repeat the process as many times as there are logistics points to move the replacements to the units. The replacements in excess of the single sub-unit I initially created do not cost metal, IP, or recruits to produce. To generate the free resources and population I just need to scrap all of the replaced models that are now sitting in my SHQ. I can then repeat this process using one old model in the SHQ to replace all of the new models contained in my fielded units, then again with the new model replacing the old, and so on. If I keep one sub-unit of the replaced model around the only cost in the entire process is the initial 100 recruits and however much metal and IP it cost to make the very first replacement.
The key to this exploit seems to be first setting it up so that you can replace 1 instance of a model (so that the "mass replace" button is no longer greyed out), then dragging the slider all the way to the right and, without releasing the left mouse button, mousing over the "mass replace" button. Upon releasing the left mouse button now, the "mass replace" button will still be clickable and the slider will remain full. If you do this with the box checked for every unit under the same SHQ you can replace hundreds of sub-units for free and gain tens of thousands of population and metal back out of it.
I've recorded a video of the process if the text explication is not clear enough, though I cannot link it directly as I have not been active on the forums enough.