As you may have seen, Google is migrating its Push Notification System.
Google Developer guide for GCM
I guess I am not alone wondering : are the tokens obtained from C2DM still valid for GCM ?
If not, it means that I need all my users to update my app with a new version updating the tokens on my servers ...
OR
I can keep the parallel systems which is something I don't think it's a good solution
OR
Continue using C2DM until it's finished, then I die with it :-)
Although client side migration is easy (just change the 'sender' from email address to a project id), you will still face transition headaches if your app has been using C2DM for a while. See my question here: Posting GCM notification to existing C2DM registration ids
Basically the problem is during the transition, you need to maintain device registration ids from the old C2DM app, and the new reg ids from your newer app that uses GCM. Unless you can force all your users to upgrade all at once, you have to build additional logic (i.e. add a new column in the database table to indicate GCM reg id) in the server to deal with sending notifications to both C2DM and GCM for a foreseeable future.
If your server only send notifications to all devices, then this should be easy migration, as you can just blast both C2DM and GCM notifications to all devices in your database, and progressively remove stale or those NotRegistered devices from the old C2DM registrations. As time goes by you should see less and less C2DM device registration ids in your database.