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If you change the stream cadence's first cast to March 23rd, those in the stream who received Email #1 will then receive Email #2 in the stream on March 23rd and those who did not receive Email #1 will receive Email #1 on March 23rd. Engagement streams will not send the same stream content to the same lead more than once.
What I would do is create a separate batch smart campaign that pauses the leads who have already received Email #1, so that on March 23 when the next cast is run, only those who did not receive Email #1 will have a normal cadence in the stream and receive Email #1. Then un-pause those leads after March 23rd and all leads will have received Email #1 and are waiting for the next cast where they will all receive Email #2 at the same time.