You can make any needed edits to your Message Templates at any time. Those changes will take effect and be reflected in what the recipient will receive at the time that the System Administrator distributes them from the User Messages queue, even if the message had been triggered prior to the edits to the template. 

If you have made an update to a Banner, even if the Banner has already been queued or distributed (into the applicant's portal), the applicant will see the updated information if they had not already signed in between the time the original Banner was triggered and updated. For example, if a Banner was distributed on January 1, and was updated on January 2, an applicant logging in after January 2 would see the updated information; but if they had logged in on January 1, they would see the "old" information.