"Replace" means it will overwrite the destination file without prompting if it already exists.
If the destination doesn't exist already, it makes no difference.
The source file is never touched, unless the conversion is writing the output to the same name and path as the input.
Put another way, "Replace" does not mean "delete the source file, even if it's distinct from the destination file". You'd need to use the Delete command after the conversion to delete the source file. (Try on a test folder first to make sure it works how you expect!)
Forgot to say I was in a single lister, Using Images from the Styles Tabs
I was assuming that HERE
Write converted images to the source folder. Without this argument converted images are written to the current destination folder.
Have tried with Single and Dual lister and they work differently
If using 1 lister it does as it says "Has to"
If using 2 listers in Detail it writes to the destination
But if source lister is in thumbnails it writes to the source lister "Strange"
Have tried adding Delete and it works correctly
Am making this for a friend so wanted one to keep Original and one to delete so he can pick which he wants