If your weapon has a lucky stat... it will have a higher probability of being upgraded compared to weapons that doesn't have lucky. Of course it's not a guarantee that you won't fail in upgrading... and doesn't mean that you don't need to add lucky powder
O btw, I dunno if having luck as a stat... stacks up to your luck when you use a lucky powder... I think it's just the chances of doing a successful upgrade at a particular time... since at times when I do upgrades with items with luck stat... a successful upgrade didn't deminish the stat... but consumed the lucky powder (since it's an upgrade)... at times... when there is a successful upgrade... the luck stat in the item is used up... Well, maybe it means during those times... instead of a failure... since the item had luck... it did an upgrade
dunno... maybe someone can clarify this hehehe
