If you were looking for an architect to revisit a building, Rem Koolhaas would not be your first choice. Not if you wanted it treated with respect. His proposed addition to Marcel Breuer’s original Whitney Museum would have hovered above it with predatory menace, like an 11-story concrete praying mantis. When the Los Angeles County Museum of Art hired him to remodel its campus, he declared he had no interest in the “renovation of uninspired buildings” but would concentrate on revamping LACMA’s image. Such highhandedness did not sit well with potential donors; both projects came to grief. And so it is a surprise to turn to New York’s New Museum, which has been expanded in a way that shows something rare for Mr. Koolhaas: deference.