DropBox API auth error - {".tag" = "expired_access_token"}

I'm not sure understand what you mean what you mean by "temporary code". The authorization flow with authorizeFromControllerV2 works effectively the same as it did with authorizeFromController, since the SDK handles the refresh flow for you. Just like with authorizeFromController, you can just process the authorization flow with authorizeFromControllerV2 and let the SDK handle the access token details for you. You would check and use authorizedClient the same way in both cases. (While authorizeFromControllerV2 uses short-lived tokens in its implementation, it works the same way for the app's code as before, since it maintains long-term access using refresh tokens automatically just like the SDK previously maintained long-term access using long-lived access tokens from authorizeFromController.)

If you want to have the user switch to a different account, you can call authorizeFromControllerV2 again and it will replace the authorizedClient.

I recommend following the instructions in the current documentation. If something isn't working as expected/documented, please show both the steps/code you're using, as well as the full unexpected error/output.