Chapter 7 trustee filed preference action, which defendants moved to dismiss as untimely under 11 U.S.C. § 546(a). Noting that the roles of chapter 11 and chapter 7 trustees are very different, the court held that trustee had two years after the date of his appointment as chapter 7 trustee within which to file his preference complaint, even though he had served as chapter 11 trustee prior to conversion of debtor's case.