What would be better for SERP - changing just folder upper to lower case or changing the page name too?
None of the above -- You're not going to rank better due to this change.
Personally, if I did it [I'm not sure I would], I would do it for URL readability, making sure links aren't accidentally made with a [space] rather than a _ and to get the extensions off the site, because the changes you're wanting to make aren't going to make a ranking difference.
If your URLs were: /123/xyb and your page was about fluffy cats, you'd probably do a bit better with /cats/fluffy, but even then it's only going to help you if you're so far out of the top 10 the few places you might gain aren't going to make any traffic difference.
What does Accept-Ranges mean?
It means the client can request only a portion of a document rather than the whole document -- EG a single page of a PDF rather than the entire PDF file.
Interesting about some of the headers there -- I haven't seen some of them on any of the hosts I've used unless they're set manually in the PHP file -- Namely, expires + content-length, and sometimes cache-control. The content length being there is likely because the gzip is running after the PHP has generated the content and generates a content-length header.
Did you make those requests with an empty cache?
And which headers are you setting rather than a standard "I don't know Apache/htaccess at all" server configuration would and/or are you setting any via PHP?
Also, are you running a php.ini with expose PHP turned off to get rid of the X-Powered-By purposely?
BTW: You're still missing a last-modified date and/or eTag for if-modified-since or if-none-match headers to be compared to for a 304 response to be served -- Last-modified is usually served as [current time stamp] if it's served for a dynamic page.
If you go here: [freetools.webmasterworld.com...] and run it on this page [meaning the URL in the address bar right now], you'll see a much more standard response for a dynamic page with a static extension.