Thanks for reaching out.
RSS has limited support for updates (and none for deleting). It was designed for a "latest news" model, where a limited number (20-50 typically) of recently published articles is provided, not the full timeline.
In the "poll" model, RSS pollers are expected to fetch RSS feeds regularly, use GUIDs to identify new entries not already fetched, and update existing entries (based on that same GUID) if the content has been updated. If an old entry needs to be updated it would have to be "re-published" in the list of new entries, using the same GUID to avoid creating duplicates.
The "push" model (PubSubHubBub) behaves much the same way, except only new and updated articles are pushed to registered servers, which is a lot more efficient and fast.
In theory, you could provide the entire timeline in your RSS feed. In practice, this wouldn't scale if you have hundreds of articles, which would produce a gigantic XML file. Feedly pollers typically truncate very large feeds and do not process the full dataset.
So back to your question: if you re-publish articles, the content in Feedly should automatically be updated, assuming that the GUIDs didn't change, the image links were updated, and the Feedly server detects that the content changed. You will need to work in batches (40-50 articles at a time), and wait for the content to be polled/pushed before you move to the next batch.
In any case, I highly recommend testing the whole process with a single article first, to make sure it gets updated and there is no duplicate article created.
I hope this helps.