All the upgrades sections are the same (default and unique ones), and must have the following sections:
id A numerical id for the upgrade. This id must unique across all the upgrades in all the ladders. Default upgrades have a default id of 0 and cannot be changed.
display The display prefix/suffix for spawners and mobs. This string is attached to names of entities and spawners.
cost The required balance to upgrade into this upgrade. Default upgrades don't need a cost section.
next-upgrade The name of the next upgrade. This section is optional, and if it doesn't exist, the upgrade will be treated as the last one in the ladder.
icon A custom icon that will be displayed inside the upgrades menu. This section is optional, and if it doesn't exist the upgrades will have a default icon.