I would have thought that they'd stop production after the new model is released (if its a direct replacement...)
The older models that are sold in the overlapping year(s) could be stockpiled vehicles that were manufactured but obviously weren't all sold before the new model took over. I could be wrong but thats what I woulda thought
If you kept on manufacturing and selling the older model it would have to be at a heavily discounted price, at the time at least, seeing as they were being replaced by an updated model.
If that makes sense