In case you missed it, Steven Halsey just posted some great info on the new Exchange Management Pack and an unsigned MP that will help with the migration over on the System Center Operations Manager team blog:
One of the scenarios supported by the new Exchange 2007 MP (6.0.6623.0) is to keep the Exchange 2007 Converted MP (6.0.6461.0) installed. This allows you to keep your historical data from the old Exchange MP, while gaining the advantages of monitoring your Exchange 2007 servers with the new Management Pack. In order to do this you need to disable the monitoring from the old MP to avoid duplication of monitoring. This can be done with the attached unsealed management pack…
To read more and to download his attachment see New Exchange 2007 Management Pack and Keeping Old Management Pack Data.
J.C. Hornbeck | Manageability Knowledge Engineer