youtube-dl changing the format output of %(upload_date)s
Solution 1:
With the perl-flavoured rename command, you could write:
rename 's/^(\d{4})(\d{2})(\d{2})/$1-$2-$3/' [0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]-*
I recommend you use the ISO standard YYYY-MM-DD date format: it is unambiguous and it sorts the same lexically and chronologically.