I was just making some swords, and then sharpening them.. Since they are a stack of the same item, it allowed me to do all of them at once.. Luckily, when i was doone, i checked the new stats, and price of the upgraded swords.. I then realized, it had only done 1 single sword.. went from 175g, to 245g worth.. only 1 of 4 did this.. So to be sure, i tried again.. Did the entire stack.. Again, it only did 1 sword.. I now had 2 of 4 upgraded..
It appears as though its onyl sharpening the 1st in the stackk that can be upgraded..