you may have to change spacers, as the hubs MAY be different. The other issue may be with your front fender, if the fender you are using now fits really close to the tire you could have a clearance issue. Beyond that when you change the wheel size you will change the trail in the front end, effecting the way it feels on the road, that is if you dont lower the front end to compensate for the new wheel.
Now all that being said going from a 19" wheel to a 21" is not a huge difference, and chances are more likely than not you will have no issues at all. I just mention these things because for every action there is a re-action, and you should be aware of what things come into play with the change you are thinking about making. Good luck and hope it works out well for you, I really like the way the 21's look on the sportys
