Pretty URLs and static pages

Hi
I have a website which consists of static html pages

made with the help of software.There are about

couple of dozen pages with the address ending in .html. Now after knowing the features

and realizing the power of blogging, I want to install

WP blog.
The problem is that almost 90% traffic comes through

Search Engine. If I install wp blog will I lose all of my Traffic and Page rank?
Someone told me that this can be solved by using pretty urls feature with the /%postname%.html. In this case my existing url https://www.example.com/test.html will become https://www.example.com/test.html (same as before). I want to know whether this is true, and what would happen to my page ranks?
thanks