What makes you think it's the same body? The empty coffin with the bandages? I'll admit that's a bit confusing, but all of the coffins are empty, so I'm not really sure what that's saying.
Anyway, Chara and Frisk are different people. Asriel knew Chara, but he died a long time ago. I doubt his body would stick around this long. Pretty sure you're controlling a fresh new body that just fell into monster land. And that person's name is Frisk.
The angel/destroyer prophecy is a neat way for them to cover both the pacifist and genocide endings at once.