Just finished reading the libretto of Graun's Britannico. I think it is a cool translation of Racine's tragedy into Italian opera.
Mostly it keeps close to the scene structure of Racine's play, with two exceptions.
(1) Unlike the play (that opens with Agrippina complaining in front of Nero's shut bedroom door), the opera starts with the embassies episode (when Agrippina wanted to ascend the throne and Nero prevented her), which was not in the play.
It allows to start off with a showy chorus scene, while at the same time establishing the shift of power from Agrippina to Nero.
(2) The last scene of the opera is the banquet at which Britannicus is poisoned: offstage in the play, it is made onstage in the opera. Due to the swiftness of the poison, Britannicus dies with no last words, which for a title character of the opera is sad.
The last words of the opera are Agrippina's, denouncing Nero, followed by the chorus that promises punishment to Nero and a blessed afterlife to Britannicus.
The narration of Junia's subsequent escape to the vestals is omitted: last we hear of Junia, "she faints offstage". This is a loss that leaves the opera's ending weakened: Junia's flight is one of the most interesting parts of the play, genius not despite but because it is a narration of offstage events (Junia has already succeeded in escaping Nero's stage and script).