I am sure that some apps will only work on the series 3. However, I have recently upgraded from an original watch which I had for more than two years. All of the health, heart rate, banking, plus a few others all worked fine on my old watch. I wore it every day and it was a great health motivator. Must close those rings every day.
My only real reason to upgrade was because I can! I gave the old one to a friend and it is still going fine. Battery still lasts a whole day.