I really don't think that the AN650 could be lowered much. The seat would probably require a modified seat pan to lower it as well. That said, there is always some talented individual out there who might prove me wrong, but I do think you would end up with compromised handling and ride characteristics, and possibly some strange ergonomics.
Better off buying a machine that fits you, and tweaking performance if you wish later on. With the 400, a Malossi variator and a performance exhaust are possible enhancements. Add some Fehling chrome trim, and you've got a scoot anyone would be proud to show off. And all that can be done a step at a time as budget allows, whereas you'd need to do the 650 mods before you'd even feel comfortable riding it.
One interesting thought. A tow-pac "minitrike" kit would allow a fairly short person to ride a 650 - since the extra wheels would hold it upright anyway. But that is a $2000 add-on, and it would definitely change the handling characteristics of the machine.
http://www.tow-pacinc.com/