// create client
config = &oauth2.Config{
	...
}
token, err := config.Exchange(context.Background(), code)
client := config.Client(context.Background(), token)

// get access and refresh tokens
t, ok := client.Transport.(*oauth2.Transport)
if !ok {
	return errors.New("cannot convert to oauth2.Transport")
}

newToken, err := t.Source.Token()
if err != nil {
	return err
}
fmt.Println("new token: %s", newToken)

if newToken.AccessToken != token.AccessToken {
	// save DB, etc...
}