Windows 2003 Domain Controller in 2000 Network

I have two machines that are clustered. We want to redo them, with Windows 2003 Advanced server.
But the main DC is a Windows 2000 server, which just houses the Global
Catalog, DNS.
The 2 node clusters host the remaining FISMO's and are DC's. Will there be a problem with me making them 2003 server and connecting to a windows 2000 DC?

Also before we demote the 2000 servers, do I have to transfer the remaining FISMO's to the remaining DC?
THanks

There's no such thing as a Windows Server 2003 domain controller in a Windows 2000 domain. Once you install a Server 2003 DC, your Active Directory becomes 2003.

You'll want to upgrade your PDC emulator first. When you put in the CD and attempt an upgrade, a utility will run checking your prerequisites. This will also tell you what needs to be done before you can upgrade.