If you don't let them know, you will regret it!
When I was in Year 10, I had a really bad year and my Psychology teacher was there for me the whole time. She left at the end of the year, and I wrote her a card and gave her a present, and we are still in contact now via Facebook. Turns out she still misses me and my class!
Trust me, if somebody has made such a change, let them know. They might be having a bad time, and to hear that they have made a difference might just be a pick-me-up!