Modifications to a 23 channel radio, illegal?

I was have a conversation with a guy I know who was telling me it’s illegal to swap out a crystal in a 23 radio to basically substitute CH 1-4 for CH 36,37,38, and 40. I’m like what? Now I know there are some crazy and outdated laws out there but didn’t some mgfs offer to send in to convert 23 radios to 40 back in the day. How could it be illegal to transmit on legal CB frequencies on a CB radio. Having a hard time getting my head around this one.

Anybody ever hear of this?