While your Amateur OCP as such doesn't expire, it means nothing unless you have a current licence, as Capt Midnight says. And I doubt you can march in to ACMA with a 40 year old AOCP and get a new licence without re-doing at least some exams. Still, they look pretty basic now: I doubt most hams these days know which end of a soldering iron gets hot. But I keep paying my licence fee, as I have since I was a teenager. The 25 wpm morse (no longer needed for an AOCP) turned out to be useful in aviation, too.