In 3 weeks Google will permanently shut down Picasa API, I used it for downloading photos of my clients. I've already modified my code to work with Google Photos API, but I have some concerns about migrating to it. Is it necessary to create new api key to use new api (as far as I know Google API key works as Picasa key for now)? Is there necessity for full authorisation of new app or can it be ommited? Is Pisaca APi somehow connected with photos API?
The Google Photos Library API is independent from the Picasa Web Albums Data API. IDs, URLs and other values returned from the Picasa API are not compatible with the Google Photos Library API.
You will also need to request new authorization from your users, as the Google Photos Library API requires different OAuth authentication scopes than the Picasa API. For example, you need to request the https://www.googleapis.com/auth/photoslibrary.readonly scope to list media from the user's library.