As mentioned earlier, file names are not vital to search engines, maybe a minimal edge but not worth changing for. For those that look, they can be more help for human visitors and to an extent, improve click through rate from search results.
If you do decide to change, best to put in place page by page 301 redirects from each old URL to the new equivalent. Search engines will generally carry forward any value in the pages/inbound links and any one following an old link or bookmark will be taken seamlessly to the new page.
Should your site be on an apache server, simple job using a .htaccess file. If you are using another system, try a web search, plenty of help.