Yes it is easy to swap, a different fit kit is all you need. I ran a 300 the last two seasons on a 2011 kit and it works fine. Even with the carb heater though I've had some icing issues. The bike is definitely nimble in the trees with an ST. But I also had the xcw 5 spd which definitely isn't the ideal tranny (I think the 6 spd xcw my buddy has on his 500 is way better). I also am plating that bike and running it as a dedicated summer bike.
For this coming winter, I picked up a new 2013 KTM 450 XCF. High revving motor with 60+ HP stock. Close ratio 5 spd. I want a dedicated snowbike so this will keep the track on year round. The XCF also has a bigger tank, wiring for lights and the option for a kick start backup to the stock estart. For me, estart is a must (as is a kicker backup in the event of a starter failure). I went out a few days without my starter working on my 300 and it was a pain in the you know what. It also has an 18" rear tire if you want to run it in summer and a 6th gear can be added cheap for those who need more top end. It's probably not the best single track bike though or high speed fire road bike.
I also like the FI because I don't want to mess with jetting twice a year, don't like spilling gas when I dump the bike and don't like mixing gas in the cold. I'm not worried about the shorter service intervals because I only run it on snow anyway.
Other bikes I seriously considered were the 500 xcw and the new husky FE 501. Chose the 450 XCF over them simply because I can live with the compromises since mine will be a dedicated snowbike. If I could only have one bike for summer and winter it would be a newer 500 for sure.
The other thing to consider is that apparently the gear ratios on the 500's are better for turboing, but I have no plans for a turbo.
JC