I am guessing that your boyfriend's attorney will pass this statement on the the Prosecutor in your boyfriend's case. The Prosecutor will more than likely want to speak with you and may want to subpoena you. They may or may not be able to depending on whether your boyfriend lists you as a witness. Like the other lawyer said, you may want to consider it. I am not sure what your es-boyfriend is facing, but if your testimony can help his case, then maybe that can be your good deed for the year! Also, it would not hurt to get your own attorney, to make sure you are protected as well.
The written statement will not be admissible at trial under ordinary circumstances. It will not be a substitute for your personal presence at trial. Nothing prohibits the prosecutor's investigator from contacting you; nothing prohibits you from refusing to speak to any investigator. However, if you are properly served with a subpoena for trial, you have to honor that subpoena.
Please give some thought to your refusal to testify. You need to consider the discomfort and aggravation being a witness can bring on someone, and weigh it against the value your testimony could provide to the jury in helping them make a decision.
If either side feels your testimony is important to the case, you will likely receive a subpoena to testify from either side. A subpoena is a court order so you will be obligated to be at the designated place at the designated time to testify. If the subpoena also requires you to produce documents, you will also be required to bring them with you. Simply providing a written statement cannot substitute live testimony because it deprives the other side the opportunity to cross-examine.
Failure to comply with the subpoena is a violation of a court order, so you may be found in contempt of court which entails possible jail time and fines.
However, even if you are required to testify, you are not obligated to make things easy for the attorney trying to elicit your testimony. You do not have to volunteer information and you can keep your answers short and limited only to what you know (no guessing or speculation). You can refuse to answer certain questions if it violates certain privileges (e.g. attorney-client privilege). And there are also certain questions that you can refuse to answer if your response might incriminate you in a crime. If a question that might incriminate you comes up, you must "plead the fifth" and refuse to answer - and you must remember to do this for every time such a question is asked.
But never under any circumstances do you lie while on the stand. If you are found to have lied while under oath, you can face criminal perjury charges.
In short, if you are subpoenaed to testify, you must comply - but only when you are subpoenaed.
This response does not constitute legal advise and is not intended to replace the benefit of obtaining full legal representation.