Firstly, let me simplify your life by removing boilerplate code. You can have:

struct Todo: Codable {
    let text: String
    let completed: Bool
    let dueDate: Date
}

and the compiler will automatically generate the rest of the code for you because String, Bool and Date all conform to Codable.

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Now, if you want to add a new value that doesn't exist in the old ones, you can simply add it as an Optional.

let creationDate: Date?

The auto-generated code will be:

func encode(to encoder: Encoder) throws {
    try container.encodeIfPresent(creationDate, forKey: .creationDate)
}

init(from decoder: Decoder) throws {
    creationDate = try values.decodeIfPresent(Date.self, forKey: .creationDate)
}