This is something we’ve been doing more of late, migrating customers from SBS 2003, SBS 2008 or even SBS 2011 over to Windows Server 2012 R2 Essentials or more interestingly, the full version of Windows Server 2012 R2 Standard with the Essentials Experience role added in. When we do this, the email side of things means that clients email needs to be migrated to either Office 365, our own Cloud Hosted Exchange or an onsite Exchange 2013 server.
Microsoft have a couple of blog posts that help out with the above migration, and they are listed here for ease of reference
The Official Guide to Migrating to WS2012R2 or WS2012 Standard with the Essentials Experience is here
In a week or so, I’ll be adding to these posts with my own experiences on moving to Windows Server 2012 R2 Essentials using a Western Digital Sentinel DS6100 as my main server. Pretty cool stuff.