I’m a returning player, and I thought I understood how the gear upgrade system works, but I missed the fine print.
Basically, if you have a higher ilevel item in a slot, you can upgrade a lower ilevel item for that same slot, (to the ilevel of the higher piece) at a great discount.
I’m a resto druid, and most of the higher ilevel items I have just happen to have a lot of crit (not great for resto druid). But I didn’t realize that I could just upgrade a slightly lower piece that has lots of haste (great for rdruid). So now I’m walking around with a lot of very nice crit gear, and have vendored all my lower-level haste gear 
.
Fine print is important.