@Grif
When you get a new phone, all you should need to do is to log onto the Google Nest app on your new phone with the same username and password you were using on your old phone.
If you successfully migrated your Nest Account to a Google Account, then you can no longer sign onto the Google Nest app using "Sign in with Nest" and instead you need to use "Sign in with Google", in which case you would then sign in with your Google Account. Note that Nest Protects still work only in the Google Nest app.
https://support.google.com/googlenest/answer/9297676?hl=en&sjid=1579737712301119216-NC#verify-migrat...
If you can't sort things out, try contacting Support:
1. Go to https://support.google.com/googlenest/gethelp.
2. Select a category/product family, and type in a brief description of the issue, then click "Next".
3. Click "Other", then click "Next step".
4. Under "Resources", just click "Next step". (Clicking one of the listed links will bring up a Help page page.)
5. Under "Contact options", choose to get a phone call or chat with the team, depending on availability in your country.
