I hope that it is not something like Vic3 buildings that have 2 separate but both useful outputs cause that's a bit of a hassle to balance usually, especially with fewer states.
BUT I can see a very good use case for changing the output of a building, namely if Ceu5ar has different goods for say small arms. I don't see why you would need to rebuild your entire arms manufacturing to switch from say a musket to a blunderbuss, so just upgrading your equipment with a, better version to replace it makes a lot of sense for a Production Method.
Ceu5ar also seems to have that luxury, seeing the large amount of different goods in the game, since, as said by others, a new good is not nearly as impactful to performance as it is for Vic3, which instead of using different goods for more advanced weapons instead abstracts that into larger production.