You can't "retract" a part of a statement you gave to the police. If what you said was true, you can be subpenaed to testify about it. If what you told the police was false, you made a false police report, and making a false police report is a crime.
Changing your story, once you gave the police a statement as to what happened could possibly result in your being charged with a crime yourself. Specifically, the crime of filing a false police report comes to mind. The prosecution could also look at possible obstruction of justice charges. If any of your earlier statements were ever made under oath you might also be looking at perjury charges. With this in mind, I will add that quite often when the police take a statement in case such as yours that they are merely taking notes and constructing what is called a "narrative" of what you said when they are putting together their police report. It is always possible that something you said was taken out of context or misunderstood by the officer when he or she was speaking to you.