I've seen a lot of confusion about this. The change ought to be a buff, at least for AP Fizz. Let's break it down:
Old W:
Passive--20-60 damage + 45% AP over 3 seconds
Active--10-50 damage + 4-8% missing health on hit
New W:
Passive--20-60 damage + 45% AP + 4-8% missing health over 3 seconds
Active--10-50 damage + 25% AP on hit
What has gotten better:
- Missing health damage now applies on any auto, not just after ability activation.
- Active now has a 25% AP ratio
What has gotten worse:
- Missing health damage will not all be dealt if the enemy is hit again before 3 seconds (the passive duration refreshes).
- Missing health damage applies over 3 seconds instead of instantly.
Note: Activating W does not turn off the passive effect. However, the damage over time from the passive will refresh on additional hits (so if you hit your opponent, then hit them again 1 second later, the last two seconds from the previous hit don't get dealt).
In a previous iteration of this post, I said the change was very nearly a straight-up buff. However, it's probably a nerf for AD Fizz because that build relied on getting a lot of attack speed to generate many, many missing-health damage procs. For AP Fizz, though, the change should be nice since he doesn't rely on W damage so much, and gets an extra AP ratio to play with. I expect that the default build will become something similar to what Keane has been playing in LCS (in one playoff game he built Abyssal Scepter/Frozen Heart/Sorcerer's Shoes/Trinity Force/Zhonya's Hourglass, then the game ended). This gives you enough damage to do good work, but also some tankiness so you don't just die for your trouble when you go in for all those autos. I think he'll make a comeback as a mid-laner and be seen a bit less top.