$ touch mlcl_1 mlcl_2 mlcl_3
$ ls mlcl_*
mlcl_1  mlcl_2  mlcl_3
$ ls | while read file; do
>   mv $file `echo "${file}" | sed 's/mlcl/mll/'`
> done
$ ls
mll_1  mll_2  mll_3
$