How do subscription groups and levels govern upgrades, and what are introductory, promotional, offer-code and win-back offers?

Predict first

When do plan changes apply, and does a promotional-offer signature survive a changed offer id?

import Foundation
import CryptoKit
import StoreKit

enum Change { case upgrade, downgrade, crossgrade }
func timing(level from: Int, _ to: Int, months a: Int, _ b: Int) -> String {
    // group level 1 is the HIGHEST level
    if to < from { return "upgrade: immediately" }
    if to > from { return "downgrade: at next renewal" }
    return a == b ? "crossgrade, same duration: immediately" : "crossgrade, different duration: at next renewal"
}
print(timing(level: 2, 1, months: 1, 12))
print(timing(level: 1, 2, months: 1, 1))
print(timing(level: 2, 2, months: 1, 1))
print(timing(level: 2, 2, months: 1, 12))

print(Product.SubscriptionOffer.OfferType.introductory.rawValue, Product.SubscriptionOffer.OfferType.promotional.rawValue, Product.SubscriptionOffer.OfferType.winBack.rawValue)
print(Product.SubscriptionOffer.PaymentMode.freeTrial.rawValue, Product.SubscriptionOffer.PaymentMode.payAsYouGo.rawValue, Product.SubscriptionOffer.PaymentMode.payUpFront.rawValue)

// the server signs a promotional offer with the subscription key: fields joined by U+2063
let key = P256.Signing.PrivateKey()
let sep = "\u{2063}"
func payload(_ offer: String) -> Data {
    ["com.example.app", "KEYID12345", "com.example.pro.monthly", offer,
     "0a1b2c3d-0000-4000-8000-000000000001", "4f2c9e8a-0000-4000-8000-0000000000aa", "1767225600000"]
        .joined(separator: sep).data(using: .utf8)!
}
let sig = try! key.signature(for: payload("WINTER25"))
print("verifies:", key.publicKey.isValidSignature(sig, for: payload("WINTER25")))
print("other offer id verifies:", key.publicKey.isValidSignature(sig, for: payload("SPRING25")))
print("DER length plausible:", (68...72).contains(sig.derRepresentation.count))

• upgrade immediately, downgrade at next renewal, crossgrade depends on duration; verifies true, other offer false
• every change applies at next renewal
• the signature still verifies after changing the offer id