Unlike word processing tools, web pages are made with a standardized language - HTML. This means that any web page that can be viewed by a web browser is written in that language. And most web page editors edit HTML without any problem.
The short answer is: if you get a new web editor, it will edit your existing web pages.
There are a few online editors that won't edit existing web pages. This is because they store all the pages for the site online along with the editor. Some of these tools will let you upload HTML to them, but others won't. Make sure that you examine the editor of any online tool before you commit.