Add Hyper-V VM DC

Hello,

I inherited a network with a single phisical DC. They have Hyper-V VMs running and was thinking of promoting one of the VMs to DC. Current forest functional level is 2008 R2 but would like to get one of the 2012 R2 machines to be the DC backup. The people here said they had some authentication issues before so they demoted the second DC. Any problems that I may run into if I go forward with my plan?

Thank you very much for the help!