Yeah, there are quite a few threads like this, but it is a good question. The truth is, most stars these days have had some kind of plastic surgery, whether fans want to believe it or not. I'm not saying all celebrities do, but yeah.
She doesn't look the same as she did when she was younger, but who does? She always didn't wear much makeup back then, and her hair changes a lot, and different hairstyles/lengths can always throw you off when looking at someone or comparing them to what they looked like a few years back.
My final point - who cares if she's had surgery or not?
If you like her singing, her looks really shouldn't matter.