Petition for writ of mandate brought by decedent's personal representative to disqualify a law firm from representing the trustees because the firm had previously represented the decedent is granted, where: 1) the parties do not dispute that a substantial relationship exists between the prior and current representations; 2) the trustees have failed to identify any reason why disqualification is not required under the circumstances of this case; and 3) the trustees failed to make a prima facie showing that petitioner waived its right to seek disqualification.