If you add and publish a new email in a sequence, the email will be queued for all existing signups in addition to new signups.
New emails may go to some contacts but not others (e.g., a contact who signed up 5 days ago will not get a new email that should be sent 1 day after signup).
Updating existing emails
If you change the subject or body of a published email, the change will apply for everyone who has not yet received the email.
If you change the schedule of a published email, the change will apply for subsequent signups. Existing contacts will receive the email under the old schedule, to prevent contacts from getting the same email more than once, or out of order.
If you reschedule a fixed date email from the past to a date in the future, it may send to some contacts but not others (no signup will receive the same email twice, so if they already received this email before, it will not be re-queued).