I've used this method to go from 2012 to 2012r2, and from 2012r2 to 2016. It is relatively quick and painless.

elevated command prompt from DC:

D:\support\adprep\adprep /forestprep

D:\support\adprep\adprep /domainprep

wait for it to replicate throughout forest and domain

launch your boot key, windows install, with updates

upgrade installation

re-check bonded NICs and LACP. remove and re-add virtual switch.

install updates (RDP)