I would highly recommend creating new records for these folks. When people leave one company and join another, their needs/challenges probably change. Different behaviors/engagement for different reasons. What someone did at one company, may be completely irrelevant at the new company. And if you base the customer experience on those prior behaviors, you may miss the mark. It's usually a best practice to start fresh here.
However, if you still want to proceed down this route, this is quite easy:
Create two new custom fields (e.g., "Email Address Old" (I used an existing field called "Email Address 2" in my example below) and "Email Address Updated"). Import the list as a static list (either in "Database" or within an operational program). During the import, map the new email address to the "Email Address Updated" field. Run a batch smart campaign that uses the static list as the smart list. And then for the flow, include a CDV flow step that takes the current email address and populates the value of the "Email Address Old" value. Add another CDV flow step that changes the main Email Address to "Email Address Updated".
NOTE: in the future, you should post Marketo-related questions like this in the Products discussion section. It will get more visibility.
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