From a press release issued by the FIM:

Road Racing World Championship Grand Prix
2004 calendar: changes of dates

The British Grand Prix will be held on the circuit of Donington Park on July 25 (instead of July 11). The date of the Rio Grand Prix has also changed; it will take place on Sunday 4 July (instead of Saturday 31 July) on the Autódromo Nelson Piquet in Jacarepaguá.