Instead of saying "Welcome to the Pokemon Center, we may restore your Pokemon to full health yada yada" My Nurse Joy (in all the centers) recently switched to this as her greeting: "Great to see you, [name]! You want the usual, right?". LOL? I'm not sure what triggers the change. Perhaps it depends of your total play time or your number of visits to Pokemon Centers.
I'm pretty sure this happens when you have four stars on your Trainer Card. On diamond, when i collected four trainer stars, the message changed like that.