If the offense is on-going, the statute of limitations is likely not running and the crime could still be charged. However, it is not clear how the circumstances you describe would constitute embezzlement. You might consider consulting with an attorney familiar with both criminal charges and the possible civil lawsuit you could file against the partners.
"Filing charges" refers to a criminal prosecution. For most felonies, the state statute of limitations is three years from the date of commission. However, for theft in the first or second degree, committed through deception, the statute of limitations is six years.
If someone "changed" a tax return, contact the IRS criminal fraud division.
This could be considered a civil matter by a prosecutor. Absent clear fraud or dishonesty, a prosecutor may consider it too iffy for a criminal case. For civil remedies, you have three years to sue from converting your money, or six years if it is a breach of a written contract. When the time starts to run can vary, if you could not have discovered it earlier, the time may not start to run until you discovered what happened.
When you speak of a "statute of limitations" I assume you are asking about the time in wihich a criminal charge can be filed. My advice to all such questions is the same: If you have information that a crime has been committed, report it to the appropriate police agency. Let the law enforcement agency and the prosecutor figure out whether any attempt at prosecution would be foreclosed by a statute of limitations. That can be a highly technical and case-specific decision, and it is the prosecutor's worry, not yours.