Riot is planning to make small changes.
They say that, and at the same time they say they're planning on making changes big enough to have a clear reason to pick Vel'koz over Xerath, Lux or Ziggs. I just don't see how both can be true at the same time, while also maintaining his balanced state.
Let's assume that they're modifying his passive and replacing his W. I can't remember right off where the post was that mentioned they generally didn't like how it's being used to poke enemies through the wave while Q is such a unique ability for poking.
They can't replace W with a shield, that makes him too similar to Lux. They can't replace it with an area of zone control, that makes him too similar to Ziggs. They can't replace it with generic burst of near on demand damage, that's too similar to Xerath. So you're most likely left with some other utility/delayed damage combo. Obviously there are a lot of possible utilities that could be thrown in, both ones we already have in the game and ones we don't, but whatever it is this is going to be what defines why you pick Vel'koz. There's no way that's a small change. If it fails to become a reason to pick Vel'koz, the reason for picking Vel'koz for the mage update fails, and you're still in a situation where it would have been better to pick another mage.
The only way it works out is if Vel'koz is still considered the most balanced champion in the game and there's a distinctive situation to pick him. If both of those don't happen, then picking him was a mistake.
... Sorry if it feels like I'm going in circles, it's late and I'm tired...
. The people that read these posts and dont bother double checking the information will further spread this "vel'koz is going to be completely reworked" belief.