Conceptually, that's not really true: you just transform the pre-ES state into one or more events (in an basic accounting system, which is pretty much the simplest ES system, long-predating the name for the model, this is just creating "starting balance" entries as transactions.)
In practice, that can be challenging, but it doesn't seem fundamentally more challenging than any other legacy data conversion effort.
In practice, that can be challenging, but it doesn't seem fundamentally more challenging than any other legacy data conversion effort.