migrating within server directory

Thanks in advance. Any thoughts appreciated.
My public directory on server currently holds my current web site written as .html.( dreamweaver) – http://www.designerstitch.com.au
I have installed WP in sub directory as designerstitch.com.au/wordpress
so I could teach myself WP and play around with themes. I now have a fair to moderate understanding and am wishing to copy all content etc of current site over to new WP theme. I am concerned if I install WP into public directory along with plugins etc and start building my new site/pages (with new theme) that it will affect .html files (and subsequently google search results for customers)and also site navigation while I am making transition.
I am in no way as learned as many of you are, as my site is fairly basic but it suits my needs.
Would anyone please be able to advise me on the best solution.
Again thanks in advance for any and all thoughts.
Kind Regards
Ann