I'd be ultimately surprised if UA didn't accept it (however, this is another example why *A has been coordinating some of its fare codes). The rationale is as follows: UA has the opportunity to "sell" a C seat and be compensated by AC for almost its own price, as opposed to upgrading someone else to fill that seat. From an economic standpoint, it makes sense to accept the ticket.

That said (and rereading the original post), you probably can't change a J for an F, but only J for C. An add/collect will be required. (Interestingly, UA has recoded flights to northern Latin America on 2 cabin planes with the F cabin as C, which will make ticket swaps with MX, etc, easier).