Typically you write an in-place migration for every version, yes. Or you snapshot your working-copy database and archive the event stream up to that point, and

Typically you write an in-place migration for every version, yes. Or you snapshot your working-copy database and archive the event stream up to that point, and you play it back using the appropriate version of the processing code. It kinda sucks and there isn't a great answer. But there are a lot of use cases where there are real benefits to having that log go back to the start.