Post migration problems

Good morning,

I've recently migrated an SBS 2003 to Server 2012 Std. and Office 365.

I'm at the end and here's where the s%*t hits the fan.

My last step was to edit the DNS address on the NIC which had the SBS2003 in the primary and "loopback" as the secondary.  As soon as I remove the SBS's IP and put in the the new DC's IP the server basically stops working.  

AD stops working, can't join computers to the domain.  the FSMO shows "ERROR" and so on.

I can't seem to wrap my head around what's happening but I'm sure it's rooted in the DNS.

Any suggestions would be wonderful as the SBS is going to start shutting down soon.

Thanks so much.