GET /{user-asid}/ids_for_apps?access_token=[app_access_token]

GET /{user-asid}/ids_for_pages
    ?access_token=[app_access_token]
    &appsecret_proof=[appsecret_proof]

Both of these queries always return an empty response for a User id.

I am trying to understand this wholse PSID / ASID migration thing. - We save the User IDs retunred by our app when a user connects their login to our application to give access to their accesstoken. Does that require us to map those ids even though we only have one application? - API document says that Pages API is moving to PSID and so the ASIDs will be automatically migrated to PSID. What does that mean? Does this change affect the data returned by the pages API and the whole Oauth login flow remains unaffected for an app?