What law forces an incoming contractor to accept the inbant Collective Bargaining Agreement?

I'm a government contractor employee and was under a CBA. A new contractor has taken over the contract and refuses to recognize the CBA from it's predecessor. I've scanned the SCA and some of the NLRA but can't really find my specific answer. My new employer is using the excuse "That CBA was with them, we are here now, you have no union". I'm certain that isn't how the law is written, and a few of the different labor acts reference the successor and predecessor but I'm lost when it comes to actually finding the directive that supports my umption that a new employer has to follow the CBA that is in place or bargain for changes. This is a relatively new CBA and I'm a relatively new steward, actually I'm the only one left because the new company refused to rehire the other 5, but that's a whole different ball of wax. Pretty much any guidence will be appreciated. Thanks