migrating or transferring Primary DC as secondary DC and secondary DC as Primary DC with DNS

Need the best practices for migrating or transferring Primary DC as secondary DC and secondary DC as Primary DC.Also please guide how the DNS will work once roles are transferred

PDC - mydc1 (2008r2Windows)

SDC - mydc2 (2012r2Windows)

Want to make mydc2 (2012r2Windows) as PDC and mydc1 (2008r2Windows) as SDC

Also guide me do i need to transfer the DNS roles also i need the mydc2 (2012r2Windows) to have those roles