The best documentation I've found, very usefull is here: http://www.commodore.ca/windows/windows_2003_upgrade.htm

Some more notes: you have to upgrade the PDC/DNM/RID FSMO. But later you have to run

adprep /forestprep

This has to be done on the infrastructure master or schema master Remember there are 5 differents roles that are unique on the forest, 2 relative to the forest, and 3 for each domain:

  • domain naming master
  • infrastructure master
  • PDC
  • RID master
  • schema master