Ambassador, which competed with other Broadway landlords to woo “Cursed Child,” overhauled the Lyric at the behest of the play’s producers. A charmless barn of a theater (previously home to a series of flops, including the $75 million musical “Spider-Man: Turn Off the Dark”), it was reconfigured to feel more like an old-fashioned opera house, with a vaulted ceiling, a necklace of boxes, and 1,622 seats (down from 1,896). Even the entrance was relocated, from crowded 42nd Street to the less dense 43rd Street.

The work on the building was expected to cost about $10 million, according to documents filed with the New York City Department of Buildings.

The play, a two-part experience with a running time of more than five hours, is a sequel to the series of young adult fantasy novels written by J.K. Rowling about a boy wizard. “Cursed Child” takes place 19 years after the final book, at a time when Harry and his friends have become parents.

“Cursed Child” was written by Jack Thorne, based on a story by Mr. Thorne, Ms. Rowling and the director John Tiffany. It was developed in Britain and has been sold out in London’s West End for 22 months, and last year it won a record nine Olivier awards — the British equivalent of the Tonys — including one for best play. A third production, in Melbourne, Australia, is scheduled to open next year.

In response to questions about the show’s finances, two of the lead producers, Sonia Friedman and Colin Callender, offered a tour of the renovated theater. Strolling through the theater, they showed off phoenix sconces and dragon lanterns and a lobby wall featuring prints of patronuses (silvery animal guardians).

The color scheme is rich and dark — most of the walls are painted a color called raven plume — and a custom carpet features H monograms (for Hogwarts, Harry’s alma mater). The newly adorned exterior features giant wings and large sculptures of a child (symbol alert!) trapped in a nest.