Summary:
If you upgrade a world from before 1.9, items held in hand or worn in armor slots by mobs and armor stands get shifted and deleted.

• Items from the feet slot are moved into the mainhand
• Items from the legs slot are moved into the offhand
• Items from the mainhand slot are moved into the feet slot
• Items from the chest and head slot are deleted

Steps to reproduce:
Or you can just upgrade this world: 2024-07-04_02-07-15_equipped items.zip

1. Start version 1.8.9.

2. /summon ArmorStand ~ ~ ~ {ShowArms:1b}

3. Equip it with a full set of diamond armor and a diamond sword.
4. Get a command block (because of 100 character limit instead of 256 before 1.11).

5. /summon Zombie ~ ~1 ~ {PersistenceRequired:1,Equipment:[{id:iron_sword},{id:chainmail_boots},{id:chainmail_leggings},{id:chainmail_chestplate},{id:chainmail_helmet}]}

6. /summon Skeleton ~ ~1 ~ {PersistenceRequired:1,Equipment:[{id:bow},{id:chainmail_boots},{id:chainmail_leggings},{id:chainmail_chestplate},{id:chainmail_helmet}]}

7. Upgrade the world to the latest version.

Observed results:
The inventory of the zombies, skeletons and armor stand is shifted and some items are deleted.

Expected results:
Items should stay where they were and should not be deleted.

Screenshots: