AVG reverts to the free version automatically. Your extra features (firewall, ransomware, etc.) become disabled, but basic antivirus remains active.
Third-party sellers claiming to offer "lifetime" activation for a one-time low fee are often selling volume licenses intended for businesses or academic use. While these keys may work initially, they can be deactivated by AVG if they are found to be resold against their terms of service.
By 2015, AVG had fully transitioned to (1-year, 2-year, or 3-year plans). After Avast acquired AVG in 2016, the subscription model remained, with licenses now often redeemable across both AVG and Avast products.