discord.js/docs/custom/documents/updating.md

About the Rewrite

The rewrite takes a much more OOP approach than previous versions, which allows code to be much more manageable. It's been written completely from scratch and should be much more stable, fixing caching issues that affected older versions and it also has support for new Discord Features, such as emojis.

Upgrading your code

The rewrite has a lot of breaking changes. Major methods, e.g. client.sendMessage(channel, message) have been moved from the Client class towards their respective classes - textChannel.sendMessage(message). You can find out the full extent of these changes by looking at the classes in the documentation.

Additionally, some event names and parameters have changed - you should revisit these.