[joshfee]

I think the problem works both ways too, because for an in progress (but long-running) workflow, you may not want to retroactively apply your new business logic for the part that's already run because that would be unexpected. But for the logic that hasn't run yet, you would certainly want the latest and greatest.

I wonder if there could be an approach where you have both versions live simultaneously, and introduce some sort of "checkpoint" into the old version that would act similar to a DB migration. When re-computing a workflow you could then start from the latest checkpoint, but any workflows that were created with the old version that haven't reached a checkpoint would continue to run the old code until it does.