While Legendaries cannot be put into the upgrader currently, datamining indicates what might happen when Legendaries are able to be upgraded.
Datamining indicates that even if your Fyr'alath the Dreamrender or Nasz'uro, the Unbound Legacy were able to be put into the upgrader, there are some bugs:
As this is datamining, this could be incorrect but it may be worth double checking the cost of your Legendary when you're able to upgrade it.
The Bullion'ed Awakened Items seem to be related to these three spells in the database.
Until today at 10:00 AM PDT, the 50% Flightstone discount was only applied to IDs 360 and 382. So even if you could upgrade your Legendary yesterday, you would be paying full Flightstone cost for all of them. Thankfully, this now seems to be fixed and should be discounted as well when players are actually able to upgrade Legendaries.
Unfortunately, dataminining indicates that other upgrade discounts aren't being applied to the legendary weapons yet. There are two types of discounts from the upgrade system:
However, you'll notice something different about the ID: 405 row — it's missing an ItemGroupIlvlScalingID entry. What is