This is not correct, according to the FCC. I think I posted a deposition transcript before regarding this. The FCC's position is that if any amateur radio is modified, the person modifying it becomes a "manufacturer" and is responsible for getting the radio certified before selling it, if the radio now transmits on a service that requires FCC certification. Here is a portion of the testimony of an "expert" for the FCC.
20 Q And then, it is your understanding that
21 if he modified the ICOM and it was now capable of
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1 transmitting on CB, the regulations prohibit him
2 from selling that radio; is that correct?
3 A Yes.
4 Q Before he could sell it, he would have
5 to get it certified for every service it was
6 capable of transmitting on; right?
7 A Yes.
The basis of the FCC's position is found at 47 C.F.R. 2.909 which provides as follows: Subpart J Equipment Authorization Procedures - The following parties are responsible for the compliance of radio frequency equipment with the applicable standards: … If subsequent to manufacture and importation, the radio frequency equipment is modified by any party not working under the authority of the responsible party, the party performing the modification becomes the new responsible party.