I have an office that is running a server 2003 environment and I wanted to go to 2008. I did all the usual forest upgrades, adprep /forestprep and so on, I ran DC promo on server 2008 and it says it was all successful. I made the new server the operations master and all the other things you do for an upgrade and promo.
Prior to running to demoting the 2003 server I wanted to just power it down. To be sure the environment would run Ok. All seems to be ok except, no internet. Seems to be DNS related. I can get into firewall form outside and can remote into server form outside so I know ISP is good. Checked firewalls and internal DNS are not point to old DC. But when I power old 2003 DC back on internet is back.