If you're talking about the chain on the 150R I change my chain on a regular intervals because the 420 chain stretches so bad. I change the chain about every 6-8 weeks. By doing this I have never had to change the rear sprocket. Its when the chain streches a large amount is when you begin to get wear on the sprocket. As far as changing it remove the old chain screw your adjusters all the way up, cut chain to fit with the proper slack.
Pat, it seems that I always get myself into too many heated debates over chains and oil these days, so I try my best to let others do the teaching.
Lol...but you know me...sometimes I can't help myself. :)
As nikki stated, the first thing to do is inspect for any sort of physical deformation of the sprocket teeth. If there is none, you can re-use that sprocket. This goes a bit against the conventional wisdom that you need to change them all out as a set. But, that rule of thumb was started because most folks will not concern themselves with a chain change until they are also seeing the sprockets start to wear.
If there is any sort of hooking or deformation of the teeth at all, it is due to the previous chain wearing and growing. As the pitch of the chain grows from wear, it will force the roller into areas of the sprocket that it wasn't meant to be pushed into...and that leads to the pitch distance of the teeth to also be changed. (hope that makes sense and you can visualize it)
A new chain on a worn sprocket is also a mis-match of pitch, and will result in an accelerated wear of the chain.
If you keep your chain at a maximum of 1.5% -2% elongation from wear....which means understanding how to and practicing regular measuring of the chain pitch, then you can continue to use a sprocket. Unless the trail grunge eats a sprocket away...as it can in some cases, it can be re-used multiple times, depending on how long you let a worn chain run. Usually a sprocket will not start to show wear from a worn chain until that chain is past the 1.5% elongated mark.
Two other very important issues to remember are alignment and tension.
You must make sure you have the sprockets in perfect alignment for them to last.
You also want to have proper tension on the chain at all times. Too loose is not a good thing, but too tight is worse. Make sure you follow your manual for proper tensioning info. If you plan on buying a KTM, try to not pay attention to their chain care info. It's manual, concerning these issues, has much to be desired.