RODC in place upgrade recommendations

Hi All,

We are having a client with 20 Domain controllers in Data Center and 500 RODC's in Windows 2008 R2 OS. They want to introduce Windows 2012 R2 domain controllers in Data Center and perform windows 2012 R2 in-place upgrade for all the 500 RODC's. I would like to know the recommendations for in-place upgrade of 500 RODCs . All RODC's are in VM.

Please help on provide the best practice and recommended approaches. 

Thanks and Regards,

Hariharan