Before removing the legacy Exchange server that runs the Recipient Update Service the RUS needs to be removed from the organization. There are at least two Recipient Update Services in an Exchange organization, the Enterprise RUS and the Domain RUS (one per domain). The simplest way to remove them all is using ADSIEdit.msc.
On the Exchange Server 2007 server launch ADSIEdit.msc. Connect to the Configuration Naming Context.
Navigate to CN=Configuration/CN=Services/CN=Microsoft Exchange/CN=First Organization/CN=Address Lists Container/CN=Recipient Update Services. In the middle pane delete each of the Recipient Update Services listed.