Apply template based on if it’s an existing or a new post

I’m redisigning a web with thousands of posts and custom post types. The original site was made with a traditional theme before FSE. I’m developing a block child theme for it, I have to respect the existing content. So I need to keep certain plugins and functionality. But to give the new child theme a better user experience, I need to differentiate the existing content from the new content. My question is: Is there a way to apply a template to existing (old) posts and another template to new created posts? The two approaches I can think of are to apply a different template based on the date of creation, or to automatically assign to new created posts a certain category and automatically apply a different template to posts with that category. If someone can help with one of the two possibilities or has a different approach, I will appreciate their advice. Thanks!