Your inputs are more pronounced--so if you're abrupt at all, the 600 will amplify it. Better acceleration, better braking, better gearing. If you're sloppy and the 250 is hiding it behind a veil of sluggishness and second-rate components the 600 will be in your face about it.
Got a hamfisted throttle technique? The 600 will spin up and may buck you off or fling itself down the road out from under you.
Haven't mastered the soft-hard-soft braking and release on the front and the 600 may washout from under you or pick the back up and do a cartwheel.
Sloppy with your downshifting? If the 600 has a slipper you may not notice, if it doesn't you may be looking at some hop or skidurdle from the rear.
For the first few weeks, act like you've never ridden a bike on the 600 and go as slow as you can to learn to be SMOOOOTH on it. SMOOTH on everything, braking, acceleration, tipping the bike into a turn, picking it up out of one, shifting, starting from a standstill. Being consistently smooth is one of the hardest things you can do with a bike and if you make the initial investment to learn to be consistently smooth on each new bike you get you've bought a lot of credits toward a long and pain-free riding career.
Always learn how to ride the bike you're on and then RIDE IT--not the memory of how another bike you had handled or behaved.