Question: I have been using android c2dm push notifications for my app, but since Google is going to stop c2dm I need to know how fast I should move all my users to android GCM so as to avoid any problems faced by my app users.

Answer: 
As you can see from the notice in the [official page of c2dm](https://developers.google.com/android/c2dm/), 

> 
> Important: C2DM has been officially deprecated as of June 26, 2012.
>  This means that C2DM has stopped accepting new users and quota
>  requests. No new features will be added to C2DM. However, apps using
>  C2DM will continue to work. Existing C2DM developers are encouraged to
>  migrate to the new version of C2DM, called Google Cloud Messaging for
>  Android (GCM). See the C2DM-to-GCM Migration document for more
>  information. Developers must use GCM for new development.
> 
> 
> 

This means, you can continue using c2dm, it will work as before. But there won't be any support for it going forward. Any new features would be added to GCM.

By the way migration to GCM is pretty simple, and they have helper libraries for GCM. Move to it asap for reliable and quota-less delivery.
Google has announced end of life for C2DM notifications. The end of life date is July 30, 2015.

Found on the [C2DM home page](https://developers.google.com/android/c2dm/):

> 
> Important: C2DM was officially deprecated on June 26, 2012, and will
>  be shut down completely as of July 30, 2015. Existing C2DM developers
>  are encouraged to migrate to [Google Cloud Messaging for Android](http://developer.android.com/google/gcm/index.html) (GCM).
>  See the [C2DM-to-GCM Migration document](http://developer.android.com/google/gcm/c2dm.html) for more information.
>  Developers must use GCM for new development.
> 
> 
>