Chin Implant, Then Jaw Surgery After?

It is certainly acceptable to undergo a chin implant at this point in your lifetime, however, during any subsequent orthognathic surgery, an oral surgeon may elect to remove the implant since it will be no longer needed once the mandible has been set forward in its normal anatomical position.  

This should be discussed with your orthognathic surgeon ahead of time. They will be able to give an advise based on examination and not guessing on the internet