Tiers VI and VII
The frontal armor was enhanced for some Tier VI–VII vehicles. Thus, the number of tanks that can easily penetrate them from the front will reduce. Now, you'll have to aim at vulnerable areas or attack the flanks more often when firing at these vehicles.

Enhanced main armor:

VII
Black Prince: from 152 to 180 mm for the hull's front.
VII
IS-2: from 90 to 120 mm for the hull's front and from 100 to 140 mm for the top turret.
VII
IS: from 120 to 155 mm for the hull's front and from 100 to 140 mm for the top turret.
VII
IS-2 (1945): from 100 to 140 mm for the turret's front.
VII
KV-3: from 120 to 165 mm for the hull's front, from 100 to 145 mm for the stock turret, and from 130 to 165 mm for the top turret.
VII
T29: from 102 to 120 mm for the hull's front.
VII
T25 AT: from 120 to 145 mm for the hull's front.
VII
Jagdpanther: from 80 to 120 mm for the hull's front.
VII
Panther I: from 85 to 110 mm for the hull's front and from 120 to 160 mm for the top turret.
VII
Tiger I: from 100 to 165 mm for the hull's front.

Also, different parts of the frontal armor (cabins, hatches, cupolas, guns and their mantlets, etc.) were reinforced for the AT 8, AT 7, and aforementioned vehicles.