It will probably take 3 - 5 business days to get your laptop back if you do overnight both ways and they don't keep it for any length of time. Of course, this is excluding weekends which most people forget about.
However, simply letting them say, "we changed our warranty contract" doesn't cut it. What would keep them from selling onsite warranty service ...then simply telling you later that they don't offer onsite service anymore? It's called, "bait and switch" ...most people frown on the practice and I believe it's illegal.
If they have modified their warranty policy to not offer case replacement, then the current warranty contract on their website would need to disclose this fact. If it doesn't, they are legally obligated to honor onsite case replacement, regardless what some support person says ...or they are liable for lawsuit.
At the very least, I'd push for some kind of compensation for the fact that their policy change is causing you some inconvenience by not having your laptop for the time it takes to repair.