I would be surprised if they didn’t add in something to upgrade the axe and evoker leggo to normal fated ilvl like they did with the sylvanas bow for people who already had it.
I can almost guarantee this will be the route they take with it, I again see no reason why we wouldn’t be able to upgrade it given so far its following the logic of shadowlands.
If I were a betting man/body type 1, I’d wager quite a large sum that those of us with axes will be getting some kind of drop from Fyrakk and/or some kind of seasonal drop that upgrades it to something higher than 522.
I’d also fully expect them to carry over the existing BLP values to people.
This was the assumption ever since the Evoker legendary in 10.1. Hopefully with more Season 4 information being communicated with us Blizzard will plainly say whether this is the case or not for both Nasz’uro and Fyr’alath.
Item level 528 or 529 will most likely be the maximum item level of Myth track items in Season 4, and Fyr’alath is seven item levels above current maximum rank Myth items (on par with his Mythic raid trinkets, Augury and Rageheart). So I believe it will be either item level 535 or 536 in Season 4 if there is an item to upgrade its item level (as will his trinkets from Mythic difficulty).