It looks as though Louisiana French - like other dialects - lacks a voiceless palato-alveolar affricate.

Such speakers would naturally articulate English "chanty" as "shanty."

Spanish, another major Gulf language, does have the "church" sound. So there's no influence from Spanish.

I don't know what the ch/sh distribution might have been in any Afro-English creole that might have been spoken around the Gulf of Mexico.

It's hard to understand why English "ch" should become "sh" without some foreign influence.

In any case, it's hard for me to believe that the *dominant* pronunciation should change from "chanty" to "shanty" in just few years.