not really. turning off ipv4 on a given network would cost approximately nothing. having ipv4 while you bring up ipv6 lets you learn as you go and adapt as you learn, and is of value. there doesn’t even need to be a cutover, they coexist.
its not even like there is a significant hardware cost; all network gear for the past 15+ years has supported ipv6; you get it for free when you buy hardware that works with ipv4.
