SyntaxcreateUpgrade(    $name,    $to_product_id,    $upgrade_from=[],    $downgrade_from=[],    $special_offer_for=[],    $fallback_product_id=null,    $is_active='Y',    $buyer_readonly_keys='none‘)Creates an upgrade.Access rightsFull access required.Parametername: Name of new upgradeto_product_id: The product being soldupgrade_from: List of products separated by commas. Owners of one or more of these products can buy the product to_product_id as an UPGRADE. A change of subscription will become effective immediately.downgrade_from: List of products separated by commas. Owners of one or more of these products can buy the product to_product_id as a DOWNGRADE. A change of subscription will become effective at the beginning of the next billing period.special_offer_for: List of products separated by commas. Owners of one or more of these products can buy the product to_product_id as a MEMBER OFFER. The order of product to_product_id is not changed.fallback_product_id: If an upgrade is not possible (e.g. because the buyer does not own the necessary product), this product will be offered instead. The default is not to specify a product - then no purchase is possible.is_active: If Y, upgrade can be purchased there. Not if N. Default is Y.buyer_readonly_keys: Determines which of the buyer’s customer data should be protected (i.e. not changeable) for the purchase. Possible values are:none – None, i.e. the buyer can change email, name and addressemail – Only protectemail – buyer can change name and addressemail_and_name – Protect email and name – Buyer can change addressall – Protect email, name and address