It could mean one of a couple of things. For believers in reincarnation, It means that the person you're speaking to is convinced they've lived before, but as someone else. It's not just Buddhist religions that believe this way, but even Scientologists, too.
Another thing it could mean (and more likely) is that this is a metaphor for a complete change in the way a person's life has gone, and the person you're talking to has experienced either a dramatic or traumatic change in the way his/her life was going that they're referring to the nature of their own life before that dramatic change.
It could also be a metaphor for a radical change in career direction and the person you're talking to is making a reference to his/her professional life that drastically changed.