From prototype testing:

When a block is selected, clicking the “glyph” (H, paragraph symbol) does nothing. I wish it allowed me to change the type of block, like changing a paragraph to a blockquote, for example.

https://make.wordpress.org/design/2017/02/09/initial-gutenberg-prototype-editor-testing/#comment-25060