Is it necessary to register in the new Google Cast Developper Console the apps and devices already registered through the previous whitelisting Form ? Apps Ids seem to be shorter now. It still seems to work with previous implementations. Will it still work with the last version of the API and SDK released yesterday if I keep the same application ID ?
No - it will no longer work - your previous whitelisting was for the preview API's only. That is now history. Please re-register at https://cast.google.com/publish