Starting in '05 the frame geometry and gas tank are the same as the 07, 08 and 09 bikes so they make the easiest candidates.
The conversion should be possible with bikes from 2000 to 2004 although I don't know anybody who has tried it.
Do a search in this forum and you should be able to find more details.
Here are some fitment notes to keep in mind:
1) All shrouds from 2000 on exchange with no problems.
2) All front fender mounting holes are interchangeable but may need to be slightly shimmed to clear frame and pipe of some bikes as steering rake has changed slightly over time.
3) You absolutley have to replace the seat, airbox, subframe, rear fender and most mounting hardware.
4) You may have to replace the pipe and silencer unless you want to modify them (we just did it to an '06 so I can say it is possible.)
Be aware that you can easily spend over $800 with the subframe, seat, airbox and rear fender easily eating up most of that.