Since category and page number is dynamic, you can use htaccess and use this sample rewrite rule below:
RewriteRule (.*?)/index.(.*?)\.html $1/page/$2/
You can test the regex here – https://htaccess.madewithlove.be/ and I can see that it is working as far as I have tested it.
PS: URL Redirections are cached aggresively, you need to clear you browser cache completely to make sure that you are testing it properly.
Hope this helps!