The Unicursal Hexagram is widely believed to have been created by Aleister Crowley, but the reality is that it is a form introduced in a paper called Polygons and Polygrams, one of seven papers given out in the 4=7 grade of the Golden Dawn. Regardie did not publish this in its entirety, so it is not surprising there is some confusion, but the nature of this symbol according to the Golden Dawn is a bit different to what many Thelemites turned it into.
As the image shows, this is not just another way to draw a standard hexagram, allowing for a single united line, but rather a symbol that denotes something entirely different. It is arguably more alchemical than planetary.
It is classified as the third form of the hexangle and is called the "pseudo-hexagram" or "irregular third form." Its description is: "Denotes the presidency of the Sun and Moon over the Four Elements, united in, and proceeding from, the Spirit."
It is not, therefore, a good replacement for the hexagrams used in the Ritual of the Hexagram, as the only planets concerned are the Sun and Moon, and the Sun is no longer the central focus point, but an opposing force to the Moon, which is its sister and twin.