One thing that may help is to put <link rel=“canonical” href=“http://website.com/about/”>
in the head of the document - this tells Google the real URL of that page and the one that you want to be indexed.
If the old URLs and the new URLs both point to the same page, that shouldn’t affect your ranking - Google is usually smart enough to figure out when alternative URLs are being used.
(a) that doesn’t make any sense
(b) even if it did, it’s wrong.
Having a file extension (eg .htm or .php) is not necessary, either for SEO or, in general terms, for site architecture. Google doesn’t care if you use a page extension or what it is. If you can manage your website so that you don’t need the extension then this improves the user-friendliness of your site (people don’t need to remember which filetype you’re using) and it improves the maintainability of your site, because if you change to a different file format (eg from .php to .asp) then you won’t need to change all your links.