Alex Albon will move from Toro Rosso to replace Pierre Gasly at the main Red Bull Formula 1 team from the Belgian Grand Prix onwards.
Gasly, who has struggled to match Max Verstappen since being drafted into Red Bull as replacement for Daniel Ricciardo, will return to Toro Rosso in a straight swap.
Red Bull had previously claimed Gasly's seat was secure until the end of the season.
Anglo-Thai racer Albon has surpassed expectations in his own debut campaign with Toro Rosso in 2019.
His sudden Red Bull Racing move is billed as a chance to evaluate him for a possible full season as Verstappen's team-mate next year.
"Red Bull are in the unique position of having four talented Formula 1 drivers under contract who can be rotated between the [main] team and Toro Rosso," said a team statement.
"The team will use the next nine races to evaluate Alex's performance in order to make an informed decision as to who will drive alongside Max in 2020."
Daniil Kvyat had been considered favourite to move up from Toro Rosso had Red Bull decided to drop Gasly either during this year or ahead of next season.
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Kvyat, who was himself relegated from Red Bull to Toro Rosso mid-season three years ago to make way for Verstappen, had scored Toro Rosso's first podium since 2008 in the mixed-weather German Grand Prix last month. He has amassed 27 points to rookie Albon's 16.
While Verstappen has won two races for Red Bull this season, Gasly did not finish in the podium in his half-year with the squad and is three places and 118 points behind his team-mate in the standings.