When composing a message, switch from Plain Text to HTML format

When I'm composing an HTML message, I can switch it to plain text format. But sometimes i'm composing a message in plain text and I realize I want to switch it to HTML to add formatting. Currently, I need to copy and paste what I've written so far, close the message, and open a new message in HTML format. It would be great if I could just switch between HTML and Plaintext format with a click.