I've noticed a couple minor problems with building upgrades:
First, Celtia's unique building (Dun, replaces Walls) gives Guerrilla 1 to all units built in that city. When upgraded to High Walls, this no longer applies. I can sort of understand this, perhaps higher walls are unsuitable to provide the necessary training. I don't know if there are other similar cases.
Second, Bakery can not be built with Modern Granary, since it requires Granary. This is usually not a problem if you're aware of it, since even Pioneer gives only Granary in the new city, not the modern variant. However, to an unsuspecting player, or when capturing a city from AI, it's possible to end up with a city with Modern Granary but no Bakery. One workaround is to sell the Modern Granary and rebuild stuff in the correct order, but it's still annoying.
First, Celtia's unique building (Dun, replaces Walls) gives Guerrilla 1 to all units built in that city. When upgraded to High Walls, this no longer applies. I can sort of understand this, perhaps higher walls are unsuitable to provide the necessary training. I don't know if there are other similar cases.
Second, Bakery can not be built with Modern Granary, since it requires Granary. This is usually not a problem if you're aware of it, since even Pioneer gives only Granary in the new city, not the modern variant. However, to an unsuspecting player, or when capturing a city from AI, it's possible to end up with a city with Modern Granary but no Bakery. One workaround is to sell the Modern Granary and rebuild stuff in the correct order, but it's still annoying.