Upgrading from Server 2008 to Server 2012 R2, Server is Additional Domain Controller and File Server, What are the steps required to upgrade?

Migrating specific domain controller functionality to the new Windows Server2012 Active Directory Domain environment using new hardware, which is the best option for migrating Active Directory Domain Controller from Windows Server2008 R2 to Windows Server2012 Domain Controller.

The prerequisites for migrating to Windows Server2012 in a new hardware are as follows:1.Windows Server2008 R2 AD DS Domain and Forest functional level must be minimum Windows Server2003 or higher recommended.

2.Check the Schema version of AD DS.

3.Forest and Domains must be prepared Using adprep. In Windows Server2008 R2 Domain Controller, run adprep /forestprep and adprep /domainprep from the Windows Server2012 installation DVD \\support\\adprep folder.

I didnt tried this yet :)

it's not in the upgrade path to2012 R2, check below link to know the upgrade options to Win server2012R2

https://technet.microsoft.com/en-us/library/dn303416.aspx

I full agree with all answers

Check all Service packs and update of server2008 then, forest & domain level , 

1) First extend the schema .

2) Install the ADC