Change the file extension for files in a folder?

You don't need to open the files to rename them, os.rename only needs their paths. Also consider using the glob module:

import glob, os

for filename in glob.iglob(os.path.join(folder, '*.grf')):
    os.rename(filename, filename[:-4] + '.las')

The open on the source file is unnecessary, since os.rename only needs the source and destination paths to get the job done. Moreover, os.rename always returns None, so it doesn't make sense to call open on its return value.

import os
import sys
folder = 'E:/.../1936342-G/test'
for filename in os.listdir(folder):
    infilename = os.path.join(folder,filename)
    if not os.path.isfile(infilename): continue
    oldbase = os.path.splitext(filename)
    newname = infilename.replace('.grf', '.las')
    output = os.rename(infilename, newname)

I simply removed the two open. Check if this works for you.