Transferring FSMO Roles from Server 2003 to Server 2008 R2

I have introduced a new domain controller (2008 R2) into my network.  My main dc (2003) is in the same office.  I want to transfer the fsmo roles to the 2008 pdc.  I'm also preparing to go from exchange 2007 to exchange 2010 and feel doing this is a necessary step.  I'm aware on how to accomplish the transfer, however, I want to be aware of the impact, if any, this will have on my domain once i move them over.  I want to keep the 2003 domain controller in the environment for the next few months.  Is it ok to do this during working hours or should this wait until the weekend.  Let me know if you need more info to answer this question.

Thanks