To the best if my knowledge, there is still no in place upgrade to clustered ontap. You must migrate to different heads.

With that said, I have three different clusters... One large (4 fas 6280's) in my lab, one small(2 x fas 3240's) on my regular network and one more (2 x fas6080) used as backup or inter cluster snap mirror.

Still 8.1.1p1d1

All I can say right now is watch out for a few nfsv4/parallel NFS bugs. Other than that... It had been real nice.