What to do if I have been asked to come into the police station for questioning in a case that I was a witness for but now possibly am a suspect in?

You need to be very careful with this and I would advise you not to do so without the advice of an attorney first. The reason being is that any questions you answer for the police can and will be used against you in court if you are a defendant in the case, and even if you will only be a witness. If you make incriminating statements to the police, these statements will damage you at trial. Speak with a criminal defense attorney prior to you answering questions to the police alone and without the advice of counsel.