This short section is mostly breathing room after “Swann in Love.” Not much happens, and the writing on whole is less dense than what preceded it. Marcel is the protagonist again, and the reminiscences go back to the form of “Combray”, with less striking results.
This section does, however, contain the big twist of Swann’s Way, so:
Marcel starts going to the Champs-Elysees every day for his health, where he develops a crush on Gilberte, Swann’s daughter. His images of the Champs-Elysees and of Gilberte evoke Swann’s visions of Odette, but it’s all played more lightly, since they’re “just kids.”
(My first and still primary association with the place-name “Champs-Elysees” is the SCTV sketch “Jerry Lewis Live on the Champs-Elysees”, with Martin Short doing Lewis bits in front of a wildly approving Paris audience. I saw it when I was 11 or 12. The show was great; look at this casting from their “version” of “Death of a Salesman”:

Ricardo Montalban as Willy Loman
Margaret Hamilton as Linda Loman
George Carlin as Biff
DeForest Kelly as Happy
John Belushi as Ben

Such memories, such memories. I didn’t have the Champs-Elysees or Combray, but I had Nick at Nite.)
Anyway, Swann himself has changed too:

Swann had become to me pre-eminently her father, and no longer the Combray Swann; since the ideas to which I now connected his name were different from the ideas in the system of which it was formerly comprised, ideas which I no longer utilised when I had occasion to think of him, he had become a new, another person. (441)

The conception of shifting frames of reference and reclassifications of people into multiple conceptions was an undercurrent to this point (Swann and Odette, Swann and the Verdurins, etc.), but this is the most explicit statement of it, and it dominates the next volume much more visibly.
There’s one other shift, which is the revelation that Madame Swann is none other than Odette. Without explanation, they’ve ended up together.
What’s the point of springing such a surprise at the very end of the book? It’s not an O. Henry twist as much as it is just pulling the rug out from under the reader, and it seems arbitrary. It makes you reconsider what’s gone before, but there’s no new context in which to reconsider it. They married, that’s the end of it.
And taking Swann’s Way by itself, it is a cheat. But two hundred pages into Within a Budding Grove, it all (very impressively) makes sense.