This error message indicates that the access token you're using (client_app_key and client_app_secret in your code) doesn't correspond to the app key and secret (my_app_key and my_app_secret in your code) that you're using.
When making this case, these pairs needs to match. That is, the access token you're trying to upgrade must have originally been retrieved using an app key and secret for the same app as you're using now.
So in this case, you'll need to swap in the correct app key and secret (my_app_key and my_app_secret in your code) to be able to upgrade this access token (client_app_key and client_app_secret in your code).
We love to learn from the educators who use Dropbox. Whether you teach kids, teens, adults or a combination of all three, we want to know what apps and integrations you use with Dropbox to help with teaching. Which of the ones below is your favorite, or most used tool?