Raise Domain Functional Level

Hi to all, i have a Windows 2003 Standard edition domain controller, and the domain is configured as Windows 2000 mixed mode.
I ' ve installed a new server Windows 2003 Standard R2 SP2, and i want to promote it as second DC. The dcpromo fails and says that the source AD forest is not compatible with this server and eventually run ADPREP command tool.
Maybe pheraphs because the source forest is Windows 2000 mixed mode ? What happen if i raise the functional level ?