In SendRegning using product codes is optional, the product list primarily exists to speed the invoicing process by remembering descriptions and prices.
When sending an invoice new products are automatically added to the product list if a previously unused product code is used on that invoice. Existing products are never automatically updated based on sent invoices.
There is no connection between prices in the product list and prices on drafts or recurring invoices. Changing the price of a product only affects future lookups in the product list, no existing drafts, invoices, or recurring invoices are changed.