I don't think it's really right to say that their conversation changes because all of the move from joy to seriousness and dissatisfaction is going on in Ivan's own head.
The couple actually says very little to each other in this story. But when they are talking about estates, all is well. Ivan dreams happily of what his life would be like.
But then the whole story turns when Ivan says he wants to go abroad and his wife agrees that she would like to. For whatever reason, the thought of going abroad starts to fill Ivan's head with dissatisfied and jealous thoughts about his wife and her money.
In Chekov's "The Lottery Ticket", the husband and wife's conversation goes from joyful to serious. In the short story, Ivan Dmitritch and his wife, Masha, mistakenly believe they have won a significant sum of money in the lottery. At first, in their elation, they begin to dream aloud about all of the things they will be able to experience now that they are wealthy: getting out of debt, buying new furniture, and traveling the world. In his head, however, Ivan begins to wonder if his wife would be stingy with the money and not share it with him at all. His wife, too, begins to think about how the first person to try to take all of her winnings away will be her own husband. With doubt and hatred in their eyes, they begin to doubt the goodness in the other person. Luckily, however, they are "saved" when they realize they haven't actually won.