I don't think the upcoming IE will ever do AS much damage considering its a nerf...unless the enemy has negative armor or you have lethality
Im guessing you mean upcoming I.E vs the one that increased critical damage
But here is some math because im interested in the current one and the upcoming one.
And this is based solely around the true damage reduced to physical damage conversion and what that means for us.
T is the current true damage, P Is the total damage based on the change... (True + what would have been true but is now physical so its reduced)
T = P if armor = 0
lol more seriously though, We are looking at a 1/3rd multiplied your resistance % and the number you get is the damage that you wont take from the I.E passive after the change
So if you took 1000 Infinity edge damage and had 75% physical damage reduction, that true damage would be reduced by 333 x 75%
This is excluding everything except armor..so if they got damage amplification or you have damage reduction outside this, youll need to factor that in aswell.