How do we break an existing command already in history to multiple lines
Multiline commands always will be archived in the command history as a single line. On the terminal prompt, you cannot edit a line to appear on multiple lines.
The approach to go about this is, is to open the command in your default editor. With the cursor sowewhere in your command, hit Ctrl+XE. This opens the command in your default editor (nano on Ubuntu), where you can do all your edits, including splitting the lines. In nano, hit Ctrl+SX to save the changes and close the editor: your command will immediately be executed.