If a witness is telling 2 different stories at the preliminary hearing and another different story in the police report, is his creditability good enough to take a case to trial on a robbery when there NO other evidence besides his word? (Basically a he say she say case)
The DA can always take it to trial, it would be up to the defense to discredit the witness. But the inconsistency won't prevent it from trial.
Credibility is for the jury to decide. There is no law that forbids the D.A. to go to trial in this situation. She might decide on her own that the case is to weak, but that is a judgment call for her to make.