If the device gets a new token assigned, it will be reported as invalid w/in GCM.

After the next send, we should be receiving the invalid tokens, and clean it up here:
https://github.com/aerogear/aerogear-unifiedpush-server/blob/master/push/sender/src/main/java/org/jboss/aerogear/unifiedpush/message/sender/GCMPushNotificationSender.java#L131-L166

But again, you have to perform a send for that.