How to edit Codable data model while saving it's data - ios

Question

Hey I recently created released an app and I wanted to add some more features but the way that I saved the data of the app was in plists using Codable. The problem is that I want to add some more features and when I try to add or remove a variable in the data model it completely wipes the data. I really don't want my users to loose their data.
I have already tried making a new class that inherited the previous class, that stopped the data from being wiped but it didn't let me save anymore data. This is for an iOS app using the latest version of Swift and UIKit
my load data function
func loadClasses(){
if let data = try? Data(contentsOf: dataFilePath!){
let decoder = PropertyListDecoder()
do{
try classArray = decoder.decode([Course].self, from:
data)
}
catch{
print("error loading items \(error)")
}
}
if let data = try? Data(contentsOf: remindersDataFilePath!){
let decoder = PropertyListDecoder()
do{
try reminders = decoder.decode([Reminder].self, from:
data)
}
catch{
print("error loading items \(error)")
}
}
if let data = try? Data(contentsOf: completedDataFilePath!){
let decoder = PropertyListDecoder()
do{
try completedReminderes =
decoder.decode([Reminder].self, from: data)
}
catch{
print("error loading items \(error)")
}
}
if let data = try? Data(contentsOf: weightsDataFilePath!){
let decoder = PropertyListDecoder()
do{
try weights = decoder.decode(CourseWeights.self, from:
data)
print(weights.honors)
}
catch{
print("error loading items \(error)")
}
}
my save data function
func saveItems(){
let encoder = PropertyListEncoder()
do{
let data = try encoder.encode(classArray)
try data.write(to: dataFilePath!)
}catch{
print("Error encoding class array \(error)")
}
do{
let data = try encoder.encode(reminders)
try data.write(to: remindersDataFilePath!)
}catch{
print("Error encoding class array \(error)")
}
do{
let data = try encoder.encode(completedReminderes)
try data.write(to: completedDataFilePath!)
}catch{
print("Error encoding class array \(error)")
}
do{
let data = try encoder.encode(weights)
try data.write(to: weightsDataFilePath!)
}catch{
print("Error encoding weights array \(error)")
}
if(reminders.count>0){
for i in 0...reminders.count-1{
if(reminders.count > 0){
scheduleLocal(remind: reminders[i])
}
}
}
}
the data model I want to add more to
class oldReminder : Codable{
var name : String = "Untitled"
var completed : Bool = false
var willNotify : Bool = false
var due = Date()
var reminder = Date()
var course = Course()
let uuid = UUID().uuidString
}