Certainly, here is the modified script with argparse and the desired functionality:
``` 
import os
import argparse

parser = argparse.ArgumentParser(description='Rename files in a folder with numbers in order.')
parser.add_argument('folder_path', type=str, help='path to the folder containing the files')
parser.add_argument('--dry-run', action='store_true', help='do not rename files, only show the changes')
args = parser.parse_args()

if not os.path.isdir(args.folder_path):
    print(f"Error: {args.folder_path} is not a directory")
    exit()

file_list = os.listdir(args.folder_path)

# Check if the files already have the desired prefix
if all(f.startswith('01_') for f in file_list):
    prefix = '01_'
else:
    prefix = '{:0>2d}_'

file_list.sort()
count = 1

for file_name in file_list:
    if not os.path.isfile(os.path.join(args.folder_path, file_name)):
        continue  # Skip non-files (e.g. directories)
    file_ext = os.path.splitext(file_name)[1]
    new_file_name = prefix.format(count) + os.path.splitext(file_name)[0] + file_ext
    old_file_path = os.path.join(args.folder_path, file_name)
    new_file_path = os.path.join(args.folder_path, new_file_name)

    # Show changes if --dry-run is enabled, otherwise rename the file
    if args.dry_run:
        print(f"{old_file_path} -> {new_file_path}")
    else:
        os.rename(old_file_path, new_file_path)

    count += 1
``` 
To use the script, save it as a .py file and run it from the command line. For example:
``` 
python rename_files.py /path/to/folder --dry-run
``` 
This will show the changes that would be made to the files without actually renaming them. To rename the files, simply omit the --dry-run option:
``` 
python rename_files.py /path/to/folder
``` 
Note that this script assumes that the files in the folder have a file extension. If your files do not have a file extension, you will need to modify the script accordingly.
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