I did not have time to reproduce this in lab.
1. Start with two nodes running 10.0.23 and master->slave replication
2. Enable 4 parallel threads
3. Verify that relay-log.info contains parallel replication information
4. Stop slave
5. Disable parallel replication
6. Verify that relay-log.info still contains parallel replication information
7. Upgrade slave to MariaDB 10.1
8. Try to start replication
9. Verify that replication does not start with error message "Failed to initialize the master info structure"
10. Remove everything except first 4 lines in relay-log.info
11. Restart server (not sure if this is needed)
12. Replication works now