WordPress .htaccess redirect Case and Extension

I installed a plugin that places .htm to all pages/posts. I want to create a redirect for specific pages so that if a users enters

mysite.com/WP-SITE/alpha-POST.html

they get redirected to

mysite.com/wp-site/alpha-post.htm (this .htm exists already)

I want to do two things. 1)Redirect .html to .htm for specific pages (no need to do a general rule for all .html requests to redirect to .htm). 2) Ignore the case of the .html request.

I tried something like this:
RewriteRule ^MYPAGES/some-Page.html$ /mypages/some-page.htm [QSA,NC,R=301]

Can anyone point me to a post that discusses this or something similar or point me to useful resources? Or if anybody can help me here on the spot, that would be great! (I’ve been searching the web for almost a week for answers and can’t seem to find something that will satisfy the combination of my requests)