Why did Jim think that Joyce might be changing her name?

Mr. and Mrs. Armstrong are holding a family get-together in Wisconsin on the occasion of the 50th wedding anniversary of Mr. Armstrong’s parents. In the dialog, about fifty men, women and children from different parts of the U.S. are congregated at a picnic getting acquainted and reacquainted.
A: Hello. You must be an Armstrong, too.
B: Yes, so far. My name’s Joyce. I think everyone here is either an Armstrong or married to one.
A: Did I understand you to say you might be changing your name soon?
B: Not soon, but hope to one of these days.
A: Anyone special in mind?
B: No, not really. I’m going with someone, but it’s not that serious. What did you say your name was?
A: I didn’t say, but it’s Jim. I was named after my uncle—that man with the horn-rimmed glasses standing over there.
B: Oh, yes. I was talking to him a little while ago. His grandfather was the first one to move out from New Jersey hack in the 1800’s. Brother! Some crowd here. Say, who’s that funny-looking kid over there—the one that’s all dressed up?
A: That’s Joseph P. Armstrong IV. He’s from the Indiana branch of the family.
B: You must be from the Western branch, too.
A: That’s right, Chicago. How’d you guess?
B: Your accent gave you away. I also know your father and he lives in Chicago.
A: That isn’t fair. And you’re from Massachusetts, right?
B: Yes, it’s no good trying to hide my accent. Oh, isn’t it exciting?
A: Sure. What?
B: Oh, everything. Everybody. Look at Mr. and Mrs. Armstrong. They’re having the time of their life. Imagine—fifty years of marriage! That’s half a century.
A: Yes, they’ve seen a lot of changes in the world.
B: And such a long and rewarding family life they’ve had. You know what I’m going to do when I get back home? I’m going to make a family tree. I’ve got to be sure and talk to everybody here.
A: Oh, ooh. Excuse me. I’m supposed to be watching Joseph P. Armstrong IV. I doubt that he can handle himself with those wild little girls.
B: You’d better hurry. See you later, Jim.