Hi Kristina,
Engagement programs are pretty great and let you move the order of the emails around in an engagement stream.
As long as you're placing the email in the order you'd like, and that email was the next step for your lead, you should be fine. Since your lead is in a the wait step (wait step being 7 days?), Marketo will recognize that the lead didn't receive the next piece of content in the steam. So if you switch out Email 2 for Email 2.1, the lead will receive 2.1 instead (as long as it's approved and activated).
Hope this helps!