Nothing and everything: If you change a template that is already in use, your older articles and pages will get a new image generated from your changed template.
So nothing will be lost or broken. Every image will be made from the same version of your template. This makes it super easy to change your logo, for example ✌️
If you use our WordPress plugin, please go to Tools > Placid Social Images > Setup and press the button Delete All Cached Images after you changed a template to start the regeneration process. If you use our URL API you'll get new images automatically.