More on retiring Exchange 2003 - Public Folders
To move your public folders, open Exchange System Manager on your 2003 server. Expand the Administrative Groups and Servers. Keep expanding things until you see your Public Folder Store. I recommend performing this operation over a weekend if you have large public folders.
Right Click on the Public Folder Store and select Move all Replicas. Give this some time. Eventually, you should see your folder count on the 2003 server diminish. Running the command: Get-Publicfolder -recurse on the Exchange Management Shell should begin to show an increasingly longer list of folders.
If you receive an error or the folders just do not seem to move, there could be two things: permissions or SSL on Exchange 2007. Make sure you have full control of ALL the public folders. If you still can't move the folders, open IIS Manager on the Exchange 2007 server. Expand the server name, Web Sites, then the Default Website. Right click on Exadmin and select properties. Click on the Directory Security tab then select the Edit button on the bottom of the window. Deselect the Require Secure Channel (SSL). Click OK twice.