Well...HD-heavy undead would be affected instead of being untouchable, and CR-heavy undead would have a chance at surviving instead of being routinely one-shotted. It's the same "problem", yes, but the effects are less extreme...and it doesn't rely on those clunky mechanics either. It's an improvement on some level, at the very least.
But I apparently didn't catch your real problem earlier, and I have to question the rationale of your intent. 20HD zombies are supposed to be tough, that's why they have 20 hit dice. And their lich masters aren't as resilient, that's probably why they have zombie minions in the first place.
I find it incongruous to expect that a hulking 20HD zombie (CR 6) would be as susceptible to turning as a ghoul wizard 4 (CR 4 and +2 turn resistance). If you believe otherwise, perhaps the problem lies elsewhere in the system.