What a pain in the butt. Client has 4,203 Misc Codes (in PO) that they no longer want available to users because we're now using the I/M module - but they do want to retain Misc Codes history. And I realized you can't just disable a 'Use in __' field if no other 'Use in __' field is enabled as @KellySappington also stated. But this client does not use A/R, S/O or B/M. So what I was able to do was temporarily create A/R data so that I could enable (via VI) the 'Use in AR' setting and disable the 'Use in PO' setting. Then I removed the A/R data which luckily still left the 'Use in PO' setting as disabled. Lastly before doing anything else, I exited Sage to avoid any A/R-related errors that I initial got during testing.