I see three options :
- change on the fly all the assets in the live campaign but as you said the communication will change for contacts who have already opened emails in that campaign
- duplicate all your currents emails, updates the duplicates, and change your campaign flow to move the contacts who have never opened one of your first version emails to the new ones
- duplicate the campaign
Your choice could depend on what you monitor, the size of your campaign
Just form the technical perspective, you should pause/disable the campaign while you make edits to the assets.