Adds2008

You can add additional domain controller as Windows 2012 R2 and configure AD DS role along with DNS. Then saize/transfer roles to new domain. So you will get all users and accounts details same as earlier. Along with policies and DNS.

https://blogs.technet.microsoft.com/canitpro/2013/05/05/step-by-step-adding-a-windows-server-2012-domain-controller-to-an-existing-windows-server-2003-network/
https://blogs.technet.microsoft.com/canitpro/2014/05/27/step-by-step-active-directory-migration-from-windows-server-2008-r2-to-windows-server-2012-r2/
https://blogs.msmvps.com/mweber/2012/07/27/upgrading-an-active-directory-domain-from-windows-server-2008-or-windows-server-2008-r2-to-windows-server-2012/

when adding an additional dc(2012) to dc 2008r2 we are not able to raise the domain functional level we are getting error as in the attachment.
what should we do to resolve it.

Where is the attachment?

adds-err.jpg

You first need to raise domain functional level to windows server 2008 R2. Domain Functional Level Windows Server 2008 is not supported for windows 2012 R2. You also raise your forest functional level to resolve the error you are receiving.

https://markparris.co.uk/2010/12/07/active-directory-domain-and-forest-functional-levels-3/

than you sir

To make things clear i will give you our requirement

Present scenario
PDC- 2008r2 - ibm x226
bdc - 2008 - hcl desktop


Requiremetn

pdc - 2012  - (new hardware)
BDC - 2008r2

please tell us the best method to fulfill our requirement

Hi,

I would suggest the following in this scenario;

1) Demote the bdc - 2008 - hcl desktop (assume this isn't holding any FSMO)

2) Install 2012 R2 on the pdc - 2012  - (new hardware). Promote this server as additional domain controller onto your existing 2008 r2 domain (Ofcourse along with DNS).

3) Once you checked and confirmed all working, transfer the fsmo roles to the newly promoted server, so that this will become your Forest root DC.