Can I change to a different Editor after adding content?

Yes! =)

Which editor was used does not matter, as all of them actually pass the contents back to the original Drupal field's textarea before the form is submitted. Whether an editor was used or not is by default completely transparent to rendering or re-editing content, by design.

Naturally, an editor may format the markup a certain way, or add details a human would normally not do if typing content in manually, but editors can not do anything a human would not be able to do (with a bit more work).