upgrade_guildhall

upgrade_guildhall = {
	if = {
		limit = {
			NOT = {
				has_building_or_higher = guild_halls_01
			}
			free_building_slots = 0
		}
		add_province_modifier = extra_building_slot
		add_building = guild_halls_01
	}
	else_if = {
		limit = {
			NOT = {
				has_building_or_higher = guild_halls_01
			}
			free_building_slots > 1
		}
		add_building = guild_halls_01
	}
	else_if = {
		limit = {
			has_building = guild_halls_01
		}
		add_building = guild_halls_02
	}
	else_if = {
		limit = {
			has_building = guild_halls_02
		}
		add_building = guild_halls_03
	}
	else_if = {
		limit = {
			has_building = guild_halls_03
		}
		add_building = guild_halls_04
	}
	else_if = {
		limit = {
			has_building = guild_halls_04
		}
		add_building = guild_halls_05
	}
	else_if = {
		limit = {
			has_building = guild_halls_05
		}
		add_building = guild_halls_06
	}
	else_if = {
		limit = {
			has_building = guild_halls_06
		}
		add_building = guild_halls_07
	}
	else_if = {
		limit = {
			has_building = guild_halls_07
		}
		add_building = guild_halls_08
	}
}

As written in game/common/scripted_effects/01_building_upgrade_effects.txt.