You may continue using your existing/older AMS license key with your existing/older PDFNet SDK, as long as you have a valid license agreement with PDFTron.
The portion that does expire is the Annual Maintenance Subscription (AMS), which is what allows you to download latest versions, as well as entitles you to technical support.
New versions of PDFNet will not accept the old license key. If you upgrade to the latest version, then you should also upgrade the license key string you pass into PDFNet.Initialize() to the latest license key you received.