Legendary upgrades can be done later with Jokers

After entering Arena 6, the Pantheon opens. Then, if you level up any 5 Legendary heroes to level 8 as you get them, you'll usually have some saved passes for Celestial Heroes and about 1 Joker from the final reward of the Celestial Pass. So, level up Celestial Heroes to 8 first, and it's a waste to use others.