Changing release days is NOT good service. It's unprofessional. Good service includes being consistent and forthright. As much as Steelbook collecting is an awesome hobby it is also considered an expense, much as your monthly bills would be. So, if you plan your expenses out by a promised schedule it can cause real complications if that schedule is not adhered to. If my auto insurance debited my payment out a week early and I wasn't expecting it that would be an issue for me and they would hear about it.
Luckily, I have yet to run into a problem on Zavvi but that's mostly because I expect the unexpected with them. That shouldn't be the case. Between this and their lack of payment flexibility, it's costing them customers.
On a lighter note, I'm stoked that my Monsters Inc. is on it's way. I was getting tired of waiting for the Best Buy version to drop to a reasonable price. Now I just need an Incredilbes...