Hi,
There is a website, for example, https://apkallin1.com/ and it is built on appyn theme and has more that 100 posts. Is there any way, I can change the theme without changing the major layout of the website?
Depends on the theme if it’s using custom posts. In case, changing the theme will probably broke the website. You can test it by creating a staging website and change the theme to see how the things go