When using a design template in an existing event, it will not be possible to change any of the design settings in the event and you will only be able to upload a new header image and logo at the event level. If you upload a new header image or logo, they will replace any of the images/logo saved to the design template, but will not delete them.
If, for example, a logo has been saved in the design template, you will not only be able to replace it by uploading a new logo, but not be able to delete it at the event level.