modificar una sola linea en fichero en lenguaje c

[Eternal Idol]

MSDN:
"When a file is opened with the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten."

Tenes que abrirlo con r+ si el archivo ya existe y sino con w+.

"r+"
Opens for both reading and writing. (The file must exist.)
"w+"
Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed.

Lo de Turbo C++ lo digo por un par de razones, primero y principal es un compilador de 16 bits para MS-DOS y segundo, y no menos importante, es Borland una empresa que segun mi experiencia personal se caracteriza por desarrollar productos de baja calidad.