As you’re aware already, the IS-6 will be reworked to HD in the incoming update, and as such, its armour is getting changed. A thread on the US forums from an admin resumed the changes (the link is at the end of the article), so here they are, for those who don’t have a look at the forums 🙂
Legend :

+ improved
= didn’t change
+/- partially improved or worsened (with overall balance as result)
– worsened

Dimensions

Before the 9.14 patch :

9.14 patch and beyond :

+ Cupola has been decreased in size
+ Turret roof has been decreased in size
+ The driver’s hatch is now increased in size (this is a good thing, as you will notice in the next section)
+ The gap between the track and the fender has been decreased slightly
= The Lower glacis is decreased in size, but is also slighter weaker
= The height of the vehicle is slightly shorter
= Tracks have become noticeably bigger and now comprise external elements of the suspension. This will also make the tank easier to “track”.

Armour protection

The values displayed in the next images DO NOT factor in normalization. It should be understood that these values are given for comparison purpose only (previous/new model). They should not be used as a basis for conclusions on whether a particular gun would penetrate a certain area of the vehicle’s armor.
Before 9.14 patch :

9.14 patch and beyond :

+ Armor thickness of the top section of the upper glacis plate has been increased
+ Armor thickness of the driver’s hatch has increased significantly.
+ Armor thickness of frontal curving sections of the turret has slightly increased.
+ Thickness of the armor strip at the top section of the upper glacis plate has increased.
+ Armor thickness of the lower glacis plate has increased a bit.
+ The part of the fender, which was visible in the frontal view (marked with arrows) could be penetrated following the rules of two and three calibers, was removed.
= Armor protection of the cupolas has not changed in a noticeably fashion
= Turret inclination has improved, but it will have no effect due to the rules of two and three calibers.
= The armor strip between the lower glacis plate and the upper glacis plate has become wider, but armor thickness of the stripe has decreased.
– Armor protection of the lower section of the lower glacis plate has been decreased, due to its decreased effective armor thickness

Armour of the gun mantlet

It can be seen that the size of the embrasure behind the gun mantlet has decreased more than twofold ! Therefore, the area where the gun mantlet overlaps the turret armor has been increased significantly, with the resulting armor thickness of 300 mm or more in these sections.
Before 9.14 patch :

9.14 patch and beyond :

Turret damage model

Turret armor protection is displayed in the image, with areas of effective armor less than 220 mm highlighted—these areas can be viewed as vulnerable spots compared to the rest of the turret’s armor.
The armor protection pattern is displayed this way due to the complex structure of the surface. This variant will be easier to perceive, since we are only interested in weak spots.
It can be seen that the pre 9.14 model had one large weak spot on the left-side of the gun. The new model has two such areas—at the left and at the right of the gun, but each of them is slightly smaller than the weak spot in the previous model. Moreover, the new model does not have the weak spot on the gunner’s sight.
Before 9.14 patch :

9.14 patch and beyond :

Side armour changes

Lastly, we examine the changes made to the side of the tank int he new patch, and give the Pros, and Cons of said changes
Before 9.14 patch :

9.14 patch and beyond :

+ Main side armor plate has has been increased to 142 mm instead of 138 mm.
+ A stripe of reinforced armor has been added (screen of 56 mm + side armor of 136 mm).
+ The prominent part of the fender, which could be penetrated following the rules of overmatching, was removed.
– Effective armor thickness of the side frontal plate has decreased a bit, which is balanced by the fact that the effective armor thickness of this frontal armor has increased slightly.

Source (feedback appreciated in this thread)