I've got the same kind of FAA license, issued in 1978, based upon my Canadian license. Each time over the years I have spoken to an FAA representative, they have told me that as long as the Canadian license is valid, so is the FAA one.
I did stop by the FAA office in Rochester, Ney York, (where it was originally issued) two years ago, and they also said to keep using it. They said that it would be quite a long effort to apply for a new one, which must be done to Oklahoma City, and involves fingerprinting and similar measures.
Don't take this as the absolute fact, but it's what I've been flying with all these years.
Pilot DAR