pg_upgrade 10.2

In older versions of pg_upgrade (e.g from 9.2 to 9.3), I was able to run pg_upgrade without stopping old cluster using the check flag.

pg_upgrade -b <old-bin> -B <new-bin> -d <old-data> -D <new-data> -p 5432 -P 5434 -r -v -k -c

Note the "c" flag at the end

However pg_upgrade in 10 (I tried from 9.3 to 10.4), when I did not stop the old cluster, the upgrade failed:

***
There seems to be a postmaster servicing the old cluster.
Please shutdown that postmaster and try again.
Failure, exiting

Is this expected?

Also, when I stopped the old cluster and ran pg_upgrade with "-c" flag, the file global/pg_control got renamed to global/pg_control.old. The "-c" flag never renamed anything in the old cluster in older pg_upgrade