For the time being, C2DM still works and upgrading your app shouldn't affect it, though you are encouraged to migrate to GCM.
Android Cloud to Device Messaging (C2DM) is deprecated. The C2DM service will continue to be maintained in the short term, but C2DM will accept no new users, and it will grant no new quotas. C2DM developers are strongly encouraged to move to Google Cloud Messaging (GCM). GCM is the next generation of C2DM.