Check your inbox for a message from Trimble with your Serial Number and Authorization Code.
If your classic license no longer supports the latest OS updates or you need newer features like "Search" or "Revit Importer," moving to a subscription is the only official path. Trimble often offers "migration" discounts for users holding an old serial number who want to make the switch. If you need help, let me know: What of SketchUp are you currently running? sketchup pro classic license key
Over a 5-year period, a one-time purchase is significantly cheaper than five years of subscriptions. Check your inbox for a message from Trimble