TypeDef Struct Variable to different #define statement  during variable init.

the TI demo code provided the statement (without error). I need help to understand and fix this error.

RFFT32  fft=RFFT32_512P_DEFAULTS;  (There are 6 different default setting).

I wish to implement switch statement which setup the fft to different setting

RFFT32  fft;

case 0:
    fft = RFFT32_512P_DEFAULTS;
   break;
case 1:
    fft = RFFT32_256P_DEFAULTS;
   break;
case 2:
    fft = RFFT32_128P_DEFAULTS;
   break;

However the compiler report error "Error expected a expression".

I have tried fft = (RFFT32 ) RFFT32_128P_DEFAULTS; but no luck as it report error (RFFT32) is not allowed

The compiler is Code Composer V6 (from TI) which is based on Eclipse.

There are 6 different default based on RFFT32_512P_DEFAULTS
RFFT32_512P_DEFAULTS
RFFT32_256P_DEFAULTS
RFFT32_128P_DEFAULTS, etc

where RFFT32_512P_DEFAULTS has the following code provided by TI example code.
//------------------------------------------------------------------------------------
#define RFFT32_512P_DEFAULTS    { (long *)NULL,\
        (long *)NULL,\
        512,\
        9,\
        (long *)NULL,\
        (long *)NULL,\
        0,\
        0,\
        8,\
        (void (*)(void *))FFT32_init,\
        (void (*)(void *))FFT32_calc,\
        (void (*)(void *))FFT32_mag,\
        (void (*)(void *))FFT32_win
}
//-------------------------------------------------------------------------
The RFFT32  typedef struct has the following statement

typedef struct {  
        long *ipcbptr;
        long *tfptr;              
        int size;
        int nrstage;            
        long *magptr;
        long *winptr;
        long peakmag;
        int peakfrq;
            int ratio;
        void (*init)(void *);
        void (*calc)(void *);
        void (*mag)(void *);
        void (*win)(void *);
        }RFFT32;
typedef RFFT32 *RFFT32_handle;