When friends change, how does it make you feel and how do you react?

When friends you have known for years change personality wise, does it make you feel bad? I'm almost 30. Two of my co-workers are friends I've had for 15+ years either from high school or old jobs. They both are really nice people and easy to get along with, but they change completely depending on who is around. It's extreme changes. They don't seem to be bothered by it or notice it, but it's irritating to me. Do you think this is normal for me to feel weird? It's almost depressing! Anyone else experience this?When friends change, how does it make you feel and how do you react?
I've been through it too many times to be bothered by it. I know how I want to be treated by my friends and if the people who want to be around me can't comply, I don't need them in my life. If the change isn't one that bothers me, I just look at them and go, ';Well, huh...'; and be friends with whatever they seem to have reinvented themselves into.