Rename file extension using SystemCaller not working

Have you tried ren "$(output_folder)" *.wld *.jgw?

Also, check if the command works all right outside FME.

There should be a \ between the last part of the output folder and the *.wld, so this should work:

ren $(output_folder)\*.wld *.jgw

Also, if there's spaces in the pathname, you might need to wrap that parameter and the \*.wld in " marks

Looks like there is a space between the output folder and the file name. That should be one continuous path without spaces. If the path contains spaces, use quotes as mentioned by @oscard

 ren "$(output_folder)\*.wld" *.jgw

Fixed it with the above. You've all been helpful, thanks!