The only direct access you have to the previous value is from within a Business Rule, not a flow. The flow is something that runs asynchronously, after a record has already been updated, so the previous value is no longer available to the program/flow logic. So you are best off accomplishing this logic in a Business Rule.
That said, if you absolutely need to do this from a flow after the update is already made, you may be able to pass the values to an Event that could then trigger a Flow.
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