I have a polaris ultra 680 triple and it has a 121" track on it i want to go to a 162 with a whole new suspention i just dont know what to do for the chaincase gears, if i can use the same chain and how much lower i should gear down?
You will need to drop the gear ratio by a lot. Not only are you going with a bigger, heavier, longer track - it will have a lot larger paddles that push a lot more snow. I dropped 2 teeth on the upper gear just going from a 137x1.25 trail track to a 137x2.0 paddle track.
Your best bet would be to look at what the stock gear ratio is on a current 162" and adjust down based on the difference in engine hp between that and yours. Current 800cc engines tend to run in the 155-160hp range - I guess yours will be in the 130-140 range (sorry I am not that familiar with triples or polaris - just a wag here).
My guess is you will end up with a shorter chain. Also try to stay at or above 20 teeth on the upper gear - the smaller it is the tighter the curve on the chain which means more stress. It will wear out the chain faster and have a higher chance of breaking. With that big of a change in gear ratio I bet you will end up having to change both upper and lower gears to get the correct ratio.
If you leave the 121 gears in place you will be way over-geared. It would be similar to trying to start out in 4th gear (if you are familiar with stick shifts anyway). You would have a really hard time getting started. A lot of people gear down when they start playing off trail even when keeping the same track.
Also with that big of a mis-match between the engine power and the load on it (through the bigger track) the clutches will be working WAY too hard. You will get belt slipping and the clutches will overheat. You will blow belts like mad...
I don't know about your sled, but my belts run about $100 each. It would cost me about $200 for 2 new gears and a brand new chain.
There is a good chance you will need a new chain as well. Too short will be obvious (can't get it on there) but you don't want it running loose (will snap the chain under load) or with too sharp of a dog-leg around the tensioner (inefficient and premature wear on the chain).
It may be painful but it is usually cheaper in the long run to get the correct parts right from the start...