Compatibity tool prompts for ADPREP when upgrading windows server from 2003 to 2008 R2 domain

Trying to upgrade windows 2003 domain controller ( single server ) I kept getting prompted to upgrade my AD :

http://technet.microsoft.com/en-us/library/1d502209-cdb9-4e13-9a6c-57ad6c9d1e8b

Ended up that I had to :

1. switch to native mode.
2. run forestprep AND domainprep

After that, all was well. hope it saves someone some time.