[dozy]

Parse authenticates with GCM using the API key that you provide, as do all other services AFAIK. So, all developers need to do is use that same key in their new push provider, and their push tokens will work.

The tricky part is actually making sure that all of your push tokens that are in the Parse database are migrated to the push provider that you've chosen.

EDIT: I stand corrected, looks like you have the option to use their key as you said, my apologies! Should have figured the founder of OneSignal would know ;-)