How do you end up with event-sourcing if you use a xDD approach?

Question

When working in a TDD or BDD manner your unit tests are supposed to drive your design. But how do you end up with event-sourcing using a xDD techniques? As I see it event sourcing is something you need to adopt early on to take full advantage of it.

Lets say that you start without event-sourcing and do a release. Later on when you are releasing version 2.0 you realize that it would be great to use event-sourcing, but at that point you alread have missed all the events from version 1.0 so it makes it much harder to implement. Or do you take some kind of backup of your db from before event-sourcing and use that as base line and then add event-sourcing on top of that?