Yes, in most cases, if you receive a free upgrade to business class or first class while traveling on an economy ticket, you should be able to access the airport lounges associated with the upgraded class. However, there can be variations based on the airline's policies and the specific terms of your upgrade.

Whether an upgrade to First Class includes lounge access depends entirely on the type of upgrade and the airline's specific policy. For international long-haul flights, most major carriers (like Emirates, Lufthansa, or Singapore Airlines) will grant you lounge access even if you were upgraded using miles or a complimentary certificate, as the First Class boarding pass itself is usually the "golden key." However, for domestic flights within the United States, an upgrade to First Class often does not include lounge access (such as United Club or American Admirals Club) unless you already have elite status or the flight is a premium transcontinental route (e.g., NYC to LAX). Furthermore, if you are upgraded at the gate as a "standby" or "operational upgrade" (Op-Up) because Economy was oversold, some airlines may strictly deny lounge entry, viewing the upgrade as a seat-only benefit. Always check the "Lounge Access" section on your digital boarding pass; if it is not listed, you may be required to pay a day fee or use a credit card perk like Priority Pass to enter the lounge.