I went from the included SE version to H5, by using the license upgrade option with my C6 box.
$99 for full version Sonic, which includes free upgrade to Sonic 2. I could have stopped right there and had 2 for $99.
Its a little confusing and hidden imo, but taking the right path can save you a little money (and time, if you install the demo from disc, you can upgrade all the way with downloads).