I’ve run into this issue many times already, if I have cool looking gear and I want to extract the rune / sigil I’d have to destroy the item in order to salvage the upgrade losing the cool looking skin it feels like such a waste, same thing if you transmute the item you still lose one of the skins and a set of runes or sigils.
Why not introduce a Skin / rune / sigil extraction stone ?
The extracted skin becomes a transmutation item just like the guild armor or weapon armor transmutable items.