Wait I have a question before I upgrade my shield and sword.
When I upgrade them, does it improve them?  Like will my shield eventually become a real metal one after enough upgrades?  And my sword become non rusted and cool after enough upgrades?  Our do I need to get a new item?
Upgraded weapons will lose things that downgraded them before, or add new things on them. For example, if someone was to upgrade a bat, they might get a bat with nails in it, which could cause a bleeding affect. Or maybe you could get AP rounds for your assault rifle. You cannot upgrade a weapon so much that you get something new.
If you upgraded our shield, yes eventually it would become a small metal square shield. It would not, however, upgrade to a tower shield. That would be considered a new item.
Sorry for the cunfuzzlement