Swift 5 JSON Import with CodingKeys and ENUM values

Question

Reading in from small JSON file (cut/pasted small sample) and bring it into structs. I am using CodingKeys to change the "string key" to match what I want in the struct. I am changing the "whoresponds" key to "respondingPilot". That is all working fine. However, I also want to look at the values on that property and change on-the-fly the value brought in. For example, if I get the string "BOTH", I want to change my data that is stored to "CAPT & F/O". Same if I see "FO", I want that changed to "F/O" as it read in. Sample below reads in fine but will not make the changes. Still learning but must be something simple I am missing. Thanks!

struct CheckListsJSON: Codable {
    var name: String
    var checklistItems: [ChecklistItemsJSON]
}

struct ChecklistItemsJSON: Codable, Identifiable {
    var challenge: String
    var respondingPilot: PilotResponding

    let id = UUID()

    
    private enum CodingKeys: String, CodingKey {
        case challenge
        case respondingPilot = "whoresponds"
    }

    enum PilotResponding: String, Codable {
        case CPT = "CAPT"
        case FO  = "F/O"
        case PF  = "PF"
        case PM  = "PM"
        case BOTH = "CAPT & F/O"
    }
}

let jsonAC = "{\"name\": \"After Start\", \"checklistItems\": [{\"challenge\": \"Generators\", \"whoresponds\": \"CPT\"}, {\"challenge\": \"Isolation Valve\", \"whoresponds\": \"FO\"}}"

let fileData = Data(jsonAC.utf8)

do {
    let decodedData = try JSONDecoder().decode(CheckListsJSON.self, from: fileData)
    print("decoded:", decodedData)
} catch {
    print(error)
}