I am in the process of upgrading my Windows SBS 2008 server running Exchange, Active Directory and as a File server to Windows Server 2012 essentials. Now I know that Windows Server 2012 essentials does not have exchange, thus I was looking to migrate the Active directory and the file server to Windows Server 2012 essentials, Then I would have a separate Virtual machine running Windows server 2012 standard with Exchange 2013. Now my question is, what should I do first? Migrate the AD and File server to Windows 2012 essentials and then after the migration is finished, I create a local move of the mailboxes from SBS 2008 to Windows Server 2012 standard running exchange 2013? or should this be the other way round?
Microsoft has published information on migrating from SBS 2008 to Essentials 2012 and migrating from previous version of Windows to Essentials 2012 R2. However, it does not contain information on migrating Exchange. For that, refer to Microsoft's guide for migration from SBS 2008 to SBS 2011. While this guide doesn't exactly fit your particular situation, it will contain useful information that you can adapt as needed. You may also want to use the Exchange Server Deployment Assistant.
Note the following:
Caveat: I have not done this particular scenario, but I've done many SBS-SBS migrations and SBS-Standard server migrations.