It appears it is not possible to reapply a rendition template after a change has been made to the template (or apply a new one to images already added to the DAM), is that correct?
So for example, if I have a large batch of images loaded, and then we decide we need a new size added to the system for all of those images, they would have to be re-uploaded with the new (or edited) rendition template applied?
That is correct. However, if you would have the workflow plugin you can
apply renditions on move. In other words, set up a workflow that watched
the folder when files are being moved. Once they are moved it would
re-apply the rendition templates.