No, it's not what you might think in terms of Rita stealing Rihanna's shine.  Apparently there was some kind of issue with the filming dates that conflicted with other business ventures that Rihanna had scheduled and so, enter Rita Ora.  According to The Sun, a source stated "Rihanna impressed movie bosses with her role in Battleship. But schedules didn't work out and Rita really shone during castings. They loved her look and her attitude."

It's alright, Rihanna will more than likely get more film offers and Rita will get the chance to shine in a lane outside of being a hot up-and-coming singer.  Look out for Rita Ora in the next Fast and Furious film starring next to Dwayne "The Rock" Johnson and Vin Diesel.