In the dead of night, a 95-year-old Picasso went under the knife.

“Anything goes wrong, just stop what you’re doing,” the lead technician, Tom Zoufaly, commanded. “I don’t want to hear any screaming, yelling.”

The scene of the operation was the Four Seasons restaurant on Park Avenue, home since 1959 to “Le Tricorne,” a 19-by-20-foot stage curtain painted by Pablo Picasso. The curtain had been caught in a dispute between the New York Landmarks Conservancy, which owns the piece, and Aby J. Rosen, the owner of the landmark Seagram Building, where it resided. Mr. Rosen wanted it taken away.