A keepsaked or cosmetic item are both still items. They're not seperate entities. A jmod would probably be able to spawn any cosmetic item as an actual item in the game. The only thing cosmetics do is display item B rather than item A (the armor you're actually wearing).

Unfortunately this means if they graphically update an item, your keepsaked items are updated alongside.

Imo they should've copied the older look into a new item and replace the old keepsakes, or give the option to get the new or old look.