We have been this years hay for a while also, the only reason your told not to is the high sugar content in it! It can be more lethal than haylage for extra fizziness and tying up!
So just keep an eye out!
We have just started feeding this years hay! And some horses are becoming slightly fizzier than normal! But nothing to worry that much about... i didnt gradually add it in! Once last years hay had gone, the next day we used this years. No problems.
Like the others say, mine have been on it for a copule of months now and are fine. You just have to watc h as it is a lot 'softer' than the more stalky,older stuff.
My mare is also on this year's hay. However as I am aware of the extra fizziness (which she really doesn't need!) I still leave her out if it's quite mild as it is at the moment. She probably averages every other night in at the moment and the rest she is rugged up and left out.