Upgrading and Downgrading

I think i figured it out.

For anyone who is also confused, this helped me:

(I am using flutter)

static Future<bool> purchase(Package package) async {

try{

//this gets the packages currently subscribed to
PurchaserInfo purchaserInfo = await Purchases.getPurchaserInfo();

//then we take the entitlement id and add that into the upgradeInfo

PurchaserInfo purchase = await Purchases.purchasePackage(package,upgradeInfo: UpgradeInfo(purchaserInfo.activeSubscriptions[0],prorationMode: ProrationMode.immediateWithTimeProration));

return true;

 } on PlatformException catch (e) {
      var errorCode = PurchasesErrorHelper.getErrorCode(e);
      if (errorCode == PurchasesErrorCode.purchaseCancelledError) {
        print("User cancelled");
      } else if (errorCode == PurchasesErrorCode.purchaseNotAllowedError) {
        print("User not allowed to purchase");
      } else if (errorCode == PurchasesErrorCode.configurationError) {
        print("Config error");
        print(e);
      }

      return false;
    }

//Note this is only for one subscription at a time
}