I believe that's because Rabbit hasn't noticed that Cassie isn't quite as... Cassie-like since she came back. She's grown up.
10 seconds sound clip from The New Adventures of Winnie the Pooh - Tigger Got Your Tongue? / A Bird in the Hand (S3E10).
You can hear this line at 00:14:40.920 in the Blu-ray version of the TV Series.
Quote context
[...]
- How about we grab the rabbit and we switch him for the bird? Heh heh.
- Nope. Getting closer, though.
- Rabbit certainly is glad Cassie's back.
- Yeah, but I don't think she's so glad about it.
- Yes, Cassie, dear, it's going to be just like old times.
- I'm too old for old times. I'm grown up.
- Of course you are, dear. I just love it when you pretend you're a big girl.
- But, Rabbit, I don't take naps anymore.
[...]