Overall, I think if only the ISU is going to be changed and it's going to receive both a rear armour nerf and a range nerf with it's HE ammunition, then it should also receive mobility/speed buffs, significant reduction to switch shell types, and/or a reload increase to AP rounds. This is all to counter the vulnerabilities that are being introduced to it.
The ISU does not need to be compensated for finally receiving some long deserved nerfs.
Said nerfs will barely affect players who are already properly supporting their ISU. You'll still be able to comfortably camp a VP and bleed enemy inf, and the rear armor only comes into play if you're caught with your pants down, which is more than fair.