FCC certified module with uncertified antenna: technical or legal issue?
If you modify a pre-certified module, it then become (in my mind) a non-certified module, and you get to re-test it. That may or may not fit into your budget. 

Under FCC rules you cannot modify the pre-certified module.

A different antenna changes the gain and radiation patterns, which could also make it non-compliant to FCC standards. The FCC is kind enough to let people use pre-certified modules if they desire. It's the FCC's job to make sure devices don't interfere with each other, if you modify an intentional radiator you can bet that they'd want to find out if it's compliant or not. 


Source: https://emcfastpass.com/rf-modules/