Its not at all advicable @shoeb. You should rethink about this decision.
Content, Images, Themes together called as a blog.
Changing content, images, themes means you are entirely launching a new blog.
First, was it your top priority? If you don’t have an answer better, drop this idea.
Only those bloggers who created a BRAND like SML won’t get affected with this major change but other’s have to think more than twice before doing this.
1.) The first thing, your USER EXPERIENCE will be affected [May be Positive / Negative]
2.) Your SEO Settings may get disturbed unless you are sure what you are doing?
3.) Some plugin may not support your new theme which causes some troubles.
4.) Bring 100% change at once is not advisable, first you improve content (or) images (or) design. Do not work to improve all at once.