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?

I tried putting in a slug/URL of "file.html" but the period drops out after hitting save.

Should I manually upload file.html and have it redirect to /file?

Thanks