Minecraft wiki entry for Development_versions, associated text: UUIDs stored in NBT are now represented as an array of four integers.
Example: {UUID:[I;1498693494,1027158888,1898994005,860320107]}
Renamed a couple of fields:
OwnerUUID of tamed animals, area effect clouds, evoker fangs and projectiles is now simply Owner.
TrustedUUIDs of foxes is now Trusted.
target_uuid of conduits is now Target.