AD upgrade in-place 2008 R2 ( domain function level 2003 )

Hi all,

We have 3 DC's, all in 2003 domain function level. Two are 2008 R2 servers, one is a 2003 server. One of the 2008 R2 servers is master (FSMO). Is it possible/safe to upgrade the 2 2008 R2 systems to AD 2008 R2 and then upgrade the 2003 server to 2008 R2 and make it a DC to join the other two?

Is it as simple as adprep ( forestprep, etc ) and then dcpromo? Am I missing something obvious?

Thanks in advance.

bdemon16