This is implemented now, see:
http://www.emeditor.com/text-editor-features/history/new-in-version-14/
The Move Line Up and Move Line Down commands were added.
By default, these commands were assigned as CTRL + SHIFT + UP and CTRL + SHIFT + DOWN.
If these shortcut keys don’t work, please go to Configuration Properties, Keyboard tab, and click Reset, or assign your favorite shortcut keys to these commands.
Thank you, Yutaka.
Find me: transpose lines move line up move lines down switch lines