The below custom script should do what you need. Make a backup of your database and place the script in your nodebb folder. Then you can run it with nodeb script_name.js

/* eslint-disable no-await-in-loop */
/* globals require, console, process */

'use strict';

const nconf = require('nconf');

nconf.file({
	file: 'config.json',
});

nconf.defaults({
	base_dir: __dirname,
	views_dir: './build/public/templates',
	upload_path: 'public/uploads',
});

const db = require('./src/database');

db.init(async (err) => {
	if (err) {
		console.log(`NodeBB could not connect to your database. Error: ${err.message}`);
		process.exit();
	}

	await fixPosts();
	console.log('done');
	process.exit();
});


async function fixPosts() {
	const batch = require('./src/batch');
	await batch.processSortedSet('posts:pid', async (pids) => {
		const postData = await db.getObjects(pids.map(pid => `post:${pid}`));
		const bulkSet = [];
		postData.forEach(p => {
			if (p && p.content) {
				bulkSet.push([
					`post:${p.pid}`,
					{
						content: String(p.content).replace(/\[c\]/g, '\n```\n'),
					},
				])
			}
		});

		await db.setObjectBulk(bulkSet);
	}, {
		batch: 500,
	});
}

Much appreciated! I'll give that a run later tonight.

I am a little confused about how to run the script though. Is nodeb a typo for nodebb? If it is, I don't think i can run commands via ./nodebb [script], I just get error: unknown command as a response. If it's not a typo, what is it?

Many thanks.

@Laz it was a typo, you can run the script with node script_name.js, which invokes the node.js interpreter.