If you can rebuild and restore the Exchange 2000 server and perform a restore of the public folders database that is what you need to do. Also, do you have a public folder data base on your Exchange 2007 server? If you do not, you will need to create one. The free/busy info is contained in the schedule plus free busy system folder in Exchange 2000 and that needs to be replicated to the exchange 2007 public folders. So restore the public folder data base on a rebuild of the old Exchange 2000 server, then replicate that folder to the Exchange 2007 server. Replicate over any other folders which you need as well if that has not been done previously. Then retire the old server following proper procedures.