In the update numbered 7.9 there is supposed to be an update to the tanks, besides smaller changes to the heroes the biggest updates are supposed to be given to Sejuani, Maokai and Zac. What minor tweaks will they receive and what about another new hero?

Maokai, Sejuani, and Zac all have their skills changed, so they will receive new icons for the new skills only.

Maokai and Sejuani may receive new splash art.

When will we see Urgot's rework? Before or after the new hero? Or later?

Not before another new hero.

When will the new hero appear, before or after the mid-season changes?

Yes.

Yes means before or after?

Yes.

As mentioned earlier in this article. The statement probably indicates that we will see the new hero before mid-season changes as well as after. The 7.8 patch is the last patch before the mid-season update, so we will almost certainly see the new hero presented tomorrow.