Will older subscribers to a series of autoresponder receive messages newly added to the series? How do I schedule those messages for this purpose?

Using delay intervals will not work because this type of messages are delivered in relation to the date of contact subscription. So, a new message scheduled on day 3 (at 3 days since the day of contact subscription) will no longer be received by a contact subscribed 2 weeks ago, because day 3 be long gone by then for that person.

But you can add the new messages in the series at specific dates and times in the future. This way, the older subscriber will get them too.