I never saw that dialog box to migrate, but I've got the newest version of hangouts and the hangout dialer. With a couple of hours of my life I can never get back, I managed to change the settings in google voice and hangouts to end up with hangouts as my texting app and a combined text inbox for GV and the built in messaging app. But I still cannot figure out how to use hangouts to send a text from my GV number rather than my regular SMS phone number. I also find that when I touch a contact in hangouts, the default is to send a hangouts message, which vanishes in the ether for people not using hangouts. A couple of extra taps are needed to send an SMS instead.
If I'm missing something, please let me know.
Galaxy S3, Android 4.4.2, AT&T