Hey @mike42! If I'm understanding your question correctly, you're asking if making changes to the email template in your account will have an effect on emails that have already been sent using that template.
Once an email is in subscribers' inboxes, the content (images, copy, etc.) can't be changed. So you're fine to modify your template and it won't affect any previously-sent emails — those changes will only apply to emails going out in the future using that template ☺
Let me know if I may have misunderstood what you meant, though!