"My players are used to D&D 5th Edition but I'd like to try an Old School Renaissance-style game. Can I convert or modify 5E?"I've seen this question asked a lot. The answer is... yes. Technically.
The Right Tool For the Right JobYou're used to driving a compact automatic 2 wheel drive car on your daily commute. It's light, fuel efficient, smooth, and nimble.
You'd like to try off-road bouldering.
You could modify your commuter car. You could, given time and money and enormous amounts of effort, convert it to 4 wheel drive, rebuild the suspension, etc. You might get a decent off-road car for your troubles, but chances are you won't. Unless you undertook this project for fun or passion, the end result won't be satisfying. If you did it to save time, money, or effort, you definitely chose the wrong method.
D&D 5th EditionAs I see it, 5E has two halves. The first half is a character generation minigame. Read and buy books, pick options, imagine a future, plan a path. Usually, GMs ask people to show up with pre-built characters, so you might have weeks or even months of gestating a character in your head, getting attached, imagining them into life.
The second half is a story generation game using D&D combat rules. The goal is to generate interesting stories that highlight the characters. Deaths should be important and meaningful and tragic. Characters gain significant power, allies, contacts, and otherworldly abilities automatically by leveling. They go from heroes to superheroes.
The status quo of the world is threatened. The world is an OK place. Maybe it's not perfect, but it's not actively awful. The job of the PCs is to keep the world on an even keel. Challengers to the status quo are the enemy; chaos, orcs, cultists, criminals, madmen, supervillain schemers, and the like. Threats go from local to apocalyptic. The ideal end goal seems to be to generate a story that's really exciting to tell. People draw a lot of art of their characters and groups and of their adventures.
The rules are designed by other people. There are FAQs and patches and revisions. Official content usually takes priority over homebrewed content.
5th Edition handles this type of game extremely well.
OSR GamesThere are several great OSR Primers out there. I like Ben Milton, Steven Lumpkin, David Perry, Bryce Lynch, and Chris McDowall's Principia Apocrypha (secret V2 here).
OSR character generation isn't a minigame. In most OSR systems it's highly randomized. You don't "build" a character; you let the dice decide and deal with the results. Sometimes the numbers are good. Sometimes they're not. It rarely matters. Rolling up a new character takes a few minutes at most, and you'll need to roll up several new characters. Death is frequent and rarely glamorous.
Old-school games are not designed to generate stories. Stories will emerge, but they won't be planned or pre-written. Success depends entirely on the skill of the players in making good choices, but failure is still interesting. The world is usually ruined or disorganized. The status quo is not always worth preserving.
There are no official rulings. Your guess is as good as anyone's, most of the time.
Actually Converting 5EFirst, ask yourself why. What is the goal?
If you all wanted to run a detective story set in 1950s France, you wouldn't use 5E just because your players know it. It's not the right tool. You'd probably pick a system specifically designed to help you tell a detective story.
Are you worried about learning a new system? Don't worry. If you can run a 5E game you can run an OSR game. In many ways it's easier. If you have problems, there's a large and supportive community out there willing to assist. This thread should answer most initial questions. There's a Discord channel here and a G+ community here. Most blog authors are happy to answer questions in their comments.
I've written a free "learning dungeon" for new players. People seem to like it.
Are you worried about a complicated, fiddly, badly-written system full of obscure tables and misleading descriptions, written when RPGs were a slightly different type of wargame? Don't worry about that either. There are plenty of "new" old school games with all the modern conveniences, and the "old" ones aren't nearly the tomes you're imagining. Most people tend to run some sort of franken-system anyway.
Pop The Hood
If you still want to convert 5E to OSR, you'll need to get your hands dirty. This isn't a clean operation like changing a headlight bulb or replacing a filter. It's messy. You'll need cut, weld, and modify.
-adjust the general tone of the implied setting
-rewrite character generation to use a random method. Dan D has a decent system here. Minimal backstories, minimal "optimal" mechanical choices.
-the only way to gain XP is gold. No story milestones, no killing monsters. Just loot.
-no balanced encounters
Best of luck. There's a complete guide out there but it's 191 pages long. Seems excessive.
You may just want to try another game. Anything with "AD&D", "OD&D" or "B/X" should work as a starting point.
The Ludicrous Mechanical Compatibility of OSR GamesAs you make these changes and pull apart the system, you may start to appreciate how it was put together. Once you understand the general principles (discussed in Principia Apocrypha) you can do two amazing things:
1. Adapt all sorts of content to your games. A module, monster, or idea no longer needs the right "brand" to be used. Grab a 5th edition monster book, a strange zine, a map from a 3.5 module, and some random tables from an AD&D booklet and run a game. If you understand how to adapt things to your system of choice, anyone who makes content is now making content for you. Some content is easier to adapt of course, and your tolerance for conversion might vary from others.
2. Write new content for other people to use. Content for OSR games varies enormously. Everyone's an amateur in a way. Write the stuff you want to see in the world.