Well, Danny changed too, they just don’t make very much of it, and he changes back very quickly. He went out for track to become more desireable in Sandy’s eyes, and at the beginning of the Fun Fair, is wearing a Letterman’s Sweater, with his letter from track. The guys suddenly get serious and ask him if he’s turning Jock.
The second Sandy appears in her Slut Persona, the Letter Sweater is gone, though. And really, they do look good together in all black. But there was the attempt to preserve the play’s O. Henryish twist that he turns into a Jock while she turns into a Pink Lady.