If a new model of a unit becomes available it is possible to upgrade the production line of an older model. There are some exceptions. BC1945 cannot be upgraded to BC1948. This because between those 2 models there is the unused model of a Nuclear Battlecruiser. The same problem applies to Battleship and Heavy Cruiser. A similar problem exists about rocket interceptors. Int-V cannot be upgraded to Int-VIII before the rocket Interceptors are researched. But that might at least partially work as designed.
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