Redirect “http://old.net/category/postname” to “https://new.com/postname”?

Since your new URLs do not have any category/ in them, you can redirect every URL that contains this and follows a pattern to your new permalink structure.

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteRule ^category/(.+)$ https://www.example.com/$1 [R=301,L]
</IfModule>

Change example.com to your new website’s domain. You can also tweak with the 301 status if you wish.

Now, this

www.oldsite.net/category/postname

will be redirected to

www.newsite.com/postname

You can also add the entire URL to the rewrite rule, but since your new domain does not have /category/, It might be safe to redirect every link, unless you decide to create a post that its slug is category!