if you’re talking about the DESIGN of the DEFAULT weapon appearance (AKA aransu 9), and asking if you’ll keep that weapon appearance once you upgrade to grand celestial, then no. simply because aransu and grand celestial weapon designs are different.
but if you’re talking about weapon SKIN, you’ll retain that weapon skin even if you upgrade/breakthrough your weapon. only the effects of the weapon will change (crit, mystic, and skill modifiers, etc.)