It is not currently possible to change the planned delivery dates of already published content for New Joiners to a Campaign.
When a New Joiner is added to a Campaign, they will receive the first piece of assigned content within 24 hours.
Each additional piece of content will then be delivered following the same schedule set for the original Campaign. For example, if the original Campaign Schedule delivered content 30 days apart, this will also be true for all New Joiners who are added, at a later date, to that Campaign.
You can view the planned dates of delivery via the Campaign Schedule Report. Please see this article for further information on how to locate this: Campaign Schedule Reporting.