// ==UserScript==
// @include http://*.5ch.net/test/read.cgi/*/*
// @include https://*.5ch.net/test/read.cgi/*/*
// ==/UserScript==
(function() {
let m = location.href.match(/(https?:\/\/\w+\.5ch\.net)\/test\/read\.cgi\/(\w{2,})\/(\d{10}.*)/);
if (m) {
location.href = m[1] + "/test/read.cgi/c/" + m[2] + "/" + m[3];
}
})();

これだと新規タブで開かれてしまいます
強制的に同じタブで開くようにできませんか？