This function is used to perform batch conversion when the root URL of the original markdown file is changed.

Let's suppose, For example, you moved `https://dl.dropboxusercontent.com/u/15546257/mytory-md-content/13076-excel-to-array.md`
to `https://example.com/mytory-md-content/13076-excel-to-array.md`.

In this case, you'll need to convert `https://dl.dropboxusercontent.com/u/15546257` to `https://example.com`.
You can put the former into "Change from", and put the latter into "Change to". And then, press the "Convert" button.