As a phoenix player this is a significant nerf. Supernova lasts 6 seconds so Firespirits, which last 4 seconds, always run out. When it updates immediately now instead of lingering on (which can be quite a while considering the huge IAS slow it provides) that's a big difference.
I do like this fix but it undoubtedly weakens phoenix. Well, maybe they make up for it via some other tweaks and if not...he'll at least not become the next FotM hero.
They also fixed being able to cast sunray while using solar dive, which basically just put the ability on cd doing nothing, so far.
e:
We'll also be getting a Phoenix Immortal for TI6 or so? screeeeeeeeeeeeeeeeeeeeeee