I don’t think so no, this sword is a quest reward so no matter your expertise you will get this at gs 610.
You might be able to upgrade it to 625 but it won’t affect your expertise.
There’s better sword to spend your umbrals on tho, called Bloodsucker’s Sword, drops by Red Claw.
To upgrade a weapon with umbral, you need your own expertise at 600. The gear score of the item doesn’t matter. I discovered it with my own crafted 600 greatsword that I couldn’t push it if my expertise isn’t 600 itself.
So, it’s a bit of a theoretical question. The game meets you halfway though, so for items you find at legendary 600, or legendary 610. There is an advantage, if the rest of the perks and such are worthy, to pick the 610 during the leveling process. Because it will boost the current effective gearscore slightly further.
Yeap, well for 5 gearscore. Doesn’t matter much. Still it’s getting super easy to push expertise now to have tested it on my alt. I’d say these items are interesting. Not worth it for end game stuff but nice in the process.