Code:
$
$ cat input_3.txt
,A,FDRM0002,12/21/2017,,0.961751583,,new text/image,,,,,,,,,,,,,,,,,,,,,,,,11/29/2016,,,,,,,,,,,
$
$ perl -F"," -lane '$F[3] =~ s|(\d+)/(\d+)/(\d+)|$3$1$2|; print join(",", @F)' input_3.txt
,A,FDRM0002,20171221,,0.961751583,,new text/image,,,,,,,,,,,,,,,,,,,,,,,,11/29/2016,,,,,,,,,,,
$
$