Dragonflight Legendaries are now upgradable but as datamined, NO CREST OR FLIGHSTONE DISCOUNTS ARE AVAILABLE. Be careful if you've already upgraded a weapon to a higher item level.
Earlier today we datamined that Crests or Flightstone discounts are available when upgrading Legendaries. Now confirmed on live servers, these items are now upgradable, after logging out and downloading the Patch.
For players who upgraded another weapon expecting to get a Crest discount, don't upgrade yet because it's likely a bug.