Would it be appropriate to send them a letter?

First let me say that they will remember you. I was a college instructor for 30 years, and I remember my students. It is a wonderful idea to follow up with them and express your appreciation....it means a great deal to hear of your former students successes. I am in contact with many of my former students simply because they have bothered to contact me, and I have asked them to keep in touch. Their successes bring me great satisfaction. I would speak from the heart, and recount to them exactly what you have expressed to us here. And I'm betting they will ask you to keep in touch as you go through life.