In some cases, Nitro allows older serials to activate slightly newer (but still legacy) versions like Nitro Pro 12, though these are also reaching "sunset" status.
Nitro frequently offers significant discounts (sometimes up to 70% off) for legacy users moving to a current subscription plan. You can check your eligibility at the Nitro Upgrade Portal by entering your 18-digit serial number. Subscription Model: Nitro Pro 8 Serial 318 WORK