Do upgrades only need to be done once?

I am playing Zelda Skyward Sword and I am at a point where I could do some upgrading - but yet to do any at all.

I am wondering if any upgrades I commit to will remain for the duration of the game?

For example, if I upgrade the wooden shield and the upgraded version breaks, will I have to upgrade it again next time a buy I new one? or can I buy the upgraded version instead?

If they do need re-upgrading each time, what other items work this way?

Thanks

Almost anything you upgrade stays upgraded (such as the slingshot).

The exception to this is shields. While you will be able to re-buy each type of shield if it breaks (such as the wooden shield and the sacred shield), you will have to pay to upgrade it again (upgrading the wooden shield to the banded shield, for example).

You can, however, get your shields fixed before they break, so you won't have to go through the upgrade process each time.

Also, if you get the Sacred Shield, it will regenerate over time, so if it is close to breaking, you can always put it away for a while and it will heal itself. The same is true for its upgraded forms.)

You are upgrading an individual item, not the item class. If you had two extra quivers and upgraded one, the second one will not magically be upgraded as well. You can keep the upgraded item for as long as you keep it. Fortunately for a lot of the items you can upgrade, you keep permanently (such as your bow, slingshot, etc.).

Consuming items (such as the infused potions), destroying items (such as the wooden, metal and sacred shields) and selling items (such as the "ammo pouches") will use up/remove that item from your inventory and that includes the upgrades that are on them. You will have to get/buy that item and upgrade it again if you want it back.

It's a good idea to carry a Revitalizing Potion around, preferably the infused Plus version (which automatically restores your shield if it breaks).

The Sacred Shield(s) restore themselves over time (when they don't take a hit for a little while).

You can upgrade bomb bags, quivers, seed satchels and shields twice, and keep as many as you want (in the Item Check). A perfectionist will have about a dozen of each Large (Quiver, Bombg Bag, Seed Satchel) in the Item Check, and 10 shields.