There have been a couple previous posts on this (here and here), but it could be a tad better so hey, let's make my first bookmarklet!

Here's a single bookmarklet that works on both old and new, opted out and opted in.

javascript:(function(){if (document.getElementById('2x-container') != null) {redesign = true} else {redesign = false}if (window.location.href.indexOf('https://www.reddit.com') == 0) {main = true} else {main = false}if (redesign && main) {location.replace(window.location.href.replace("https://www.","https://old.").toString())}else if (redesign && !main) {location.replace(window.location.href.replace("https://new.","https://www.").toString())}else if (!redesign && main) {location.replace(window.location.href.replace("https://www.","https://new.").toString())}else if (!redesign && !main) {location.replace(window.location.href.replace("https://old.","https://www.").toString())}else {alert("Something went wrong!")}})()

To install, simply highlight the above and drag and drop it to your bookmarks bar. Rename it to make it look pretty too. Or stick it in your moderator toolbox bar at the bottom. (Reddit disallows links like this for perfectly good reasons so this is a little unpretty and manual).

/u/ZadocPaet suggested I make this a little Chrome extension so I can auto-update if Reddit changes something (in this case, if they rename the container used on the redesign or the subdomains) or I fucked up or for whatever other reason. There's a small registration fee but if there's interest, it seems like a neat excuse to give that a shot too.