Here is a basic timeline of what has changed since 2005.
2005 was the first year for a new frame and engine design. That basic design is still used on the 08s. The only real difference is in '07 the subframe, seat, rear fender and airbox were changed. n '08 the bike got a revised seat and new fork valving.
2006 was the first year for daul map ignition but most of the '05s got it, they just didn't get the hi-lo switch. To see if you got it pull the seat and look to how the CDI box is mounted, if it's attached to the fender, and not located in the airbox, you most likely got the '06 dual CDI box. The real test is see if the CDI box has two short wires wither plugged into each other of just loose.
Most of the '07 and '08s have a Sachs shock while the '05 and '06s most likely have a Ohlins shock.
In '08 the main bearings were changed to sealed ones.
If you want to upgrade the bike with the new 07/08 subframe, seat, airbox, and plastic you'll spend around $800 to $900 to do it.
As far as making the bike a 300 all you need is a 300 cylinder, head and piston as everything else is the same.