I have the new one and LOVE IT! I had a HillRom but rarely used it because it was uncomfortable, tubes came off, was loud, annoying, and didn't really pound the snot out of me (literally).
You should have no problem getting a new one, especially if you can get your doctor or social worker to state why it's better to have the better fitting model.
It's in the insurance company's best interest to provide you with whatever it takes to keep your daughter from losing lung function and enduring hospitalizations. The vest is a great preventive measure. I only wish I had gotten the good one much sooner.
best wishes!