OK, so if you want to switch your core back to the Spark Cloud, you’ll need to change some keys back on your core.
1.) Grab this file: https://s3.amazonaws.com/spark-website/cloud_public.der
2.) run: spark keys server cloud_public.der
3.) either use your existing key, or load back your saved key with spark keys load pre_my_old_key.der or you can try spark keys doctor.