coverting site - not losing search engine results

[atstarr]

I am converting my site from plain HTML with pages of format:

domain/subdir/file.html

to get simple, where the URLS will be more like

domain/subdir/file

how do I avoid losing search engine results?

See here on how to add .html to URLs.

Thanks. Does "root level .htaccess" file refer to:

/.htaccess
or
/public_html/.htaccess

I'm assuming first one, but wanted to doublecheck.