if you perform a fresh install you will enter the 90-days evaluation period, so the new CMX can be used for limited time
if it is traditional licensing, then from the cisco licensing portal you need to transfer the license to the new CMX., and install the new license file on the new CMX
if it is smart licensing, and you register the new CMX then new CMX will run with a license shortage
when you release the license (deregister the old CMX) the smart license will immediately come available for the new CMX.