Something like this maybe...

 Code:
for each $file in dirplus(@scriptdir)
  $newfilename="0900" + right($file.name,-6)
  ? $newfilename
next


You will need dirplus to make this work...

Dirplus -
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=82153

How to use UDFs -
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=81943#Post81943

The rest of the UDFs are here -
http://www.kixtart.org/forums/ubbthreads.php?ubb=postlist&Board=7&page=1