How to add a 2008 domain controller to an existing 2003 domain.

Here is what I have:
4 offices, each with it's own domain controller in the same domain, linked over a hardware VPN, all 2003 Standard.

I need to replace one of them with a new 2008 Standard server. I came across some instructions on the procedure for this on this site - https://www.experts-exchange.com/questions/23869071/Adding-a-Server-2008-domain-controller-to-an-existing-2003-domain.html

This may be a stupid question, I don't know, bit here it is. That link describes the steps to modify active directory to add some AD extensions that are in 2008. If I complete this on the main DC in the main office, will I also have to do this on the other three DC's? I don't know if the changes would replicate to the others, or if it has to be done manually. Also, are there any problems with running the ADPREP command through an RDP session to the remaining remote locations?

Thanks very much for any guidance you can offer.