I am having the same issue. Two of my nest products have already been set up in google home app. I bought a smoke and co alarm today and it told me I had to add it to my nest and when I scanned the barcode it made me download the nest app. I know how to migrate to the google home app but it is now doing it. It is getting very irritating as I am stuck using both accounts for my three google nest products. It is bad when you can't get help my phone with a tec support to walk you through what you have done before.
Sorry for the trouble. I wanted to follow up and see if I could be of any help. I found an article with some details on migrating your account. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.
Just checking in to make sure that you've seen our responses. Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.
I made the mistake of migrating my nest account to Google and now I cannot connect my nest account to Google home. I am also stuck using both apps. When I try to add the nest app to Google home, it takes me to the nest login and does not show an option to sign in with Google. I did find the option when I hit "forgot password" but when I select login with Google I get bad request error.