Yes, you can go right over the old stain. If there's much left of the old stain and the new one is light coloured, the new one might look a bit different than it was supposed to. A dark one, probably not.

Yes, a power washer could well remove a good deal of the old stain. Just try not to remove too much of the wood while you're at it. If the fence is cedar, it's easy to remove some wood while power washing because cedar is so soft.

it really depends on the existing stain if you were to use like behr barn red deck stain then yea no problem as it has a high volume of pigmentation

You should wash it, and the new stain may take, but it may not be a good color.

TY EVERYONE....

I stained my fence natural cedar color a couple years ago and now I want to re stain it with a different color ... Can I just go over the stain with the new stuff or do I need to do anything before I go over it with new stuff ? ? I was thinking of power washing it and hoping the power washer would strip the stain from years past.... let me know what you think ... Thanks !