This is NOT my Marketo instance, but will do for illustrating this question.
Say this is a program/controller nested in a Low Intent nurture stream. Let's call this controller 'Send Email #1 by Region'.
So a new EU lead is created, the 'add to nurture' smart campaign adds the lead to this low intent stream.
This lead gets the EU email #1. However, after the lead gets this email, his region changes to 'AP'.
Now the EU email and the AP email have different messaging. If there is a separate controller (outside the nurture) that smart list triggers when region data value changes, and flow is add to engagement program - low stream...will the lead receive the AP email now? Essentially running the same lead through the same stream twice. Is this best practice? Does the lead need to be 'paused' first, then re-added?
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I would have three streams in the nurture, one for AP, EU, and NA. In each stream place their specific content. When leads are added to the engagement program, use choice steps in the "add to engagement program" flow step to add them to the specific stream.
Have another outside smart campaign that triggers for "data value changes: region" along with the filter "member of engagement program" and use choice steps in the "change engagement program stream" flow step to change to the appropriate stream.