In all likelihood the contract will have a termination clause (they pretty much all seem to these days) so in effect MS etc. are not re-writing contracts as such, they are terminating existing contracts and offering new ones. These new ones will have the super new reduced rate of compensation.
It is highly likely that should you refuse the cut you will prejudice your chances of any future contracting / employment opportunities within Microsoft.
And as has been already pointed out as a contractor you have [virtually] no employment rights from the end client so your choice is basically take the cut or take a hike. That is one of the downsides of contracting. You pay your money you takes your chance.
As I have said before, any contractor that takes the full 10% cut is a fool, your agency should shoulder most, if not all of the cut for you. And if you contract direct then your rates will already be high enough to suck up a 10% reduction.