Can a C compiler change bit representation when casting signed to unsigned?

Is it possible for an explicit cast of, say, int32_t to uint32_t, to alter the bit representation of the value?

For example, given that I have the following union:

typedef union {
    int32_t signed_val;
    uint32_t unsigned_val;
} signed_unsigned_t;

Are these code segments guaranteed by the spec to have the same behaviour?

uint32_t reinterpret_signed_as_unsigned(int32_t input) {
    return (uint32_t) input;
}

and

uint32_t reinterpret_signed_as_unsigned(int32_t input) {
    signed_unsigned_t converter;
    converter.signed_val = input;
    return converter.unsigned_val;
}

I'm considering C99 here. I've seen a few similar questions, but they all seemed to be discussing C++, not C.