The *only* things I upgrade are the lvl 10 silver ones and the personal earrings. Why upgrade a lvl 9 when it's going to be replaced with a lvl 10 eventually? So I rather upgrade a lvl 10 necklace than a lvl 9 ring. And I have far from enough spare jewelry to upgrade all my lvl 10 jewelry, so it's not hard to "only level what I won't replace".

Upgrading a necklace with spare necklaces and upgrading rings with spare rings boosts them a bit better, but just boosting a single piece with whatever 10 spare pieces you have is more cost efficient. I tend to go for the median of that. If most of my spare pieces are rings, I use it to upgrade a ring. If most of my spare pieces are wristbands, I use them to upgrade a wristband.