Currently, you can use the 'legend.newLegend' option and set it to false in order to revert back to the old style. However, we will probably deprecate this option eventually (which is why it's undocumented). If what you're concerned about is how much horizontal space the new legend takes up, you can instead play around with the legend.iconAspectRatio (you can find an example here
http://jsfiddle.net/gw03m8to/2/) to find an aspect ratio that works for you. I imagine that down the line we could introduce some sort of 'iconStyle' option to allow you to control exactly what sort of icon appears there, but it's just an idea at this point, we don't have a roadmap for that.