In short you can do almost anything you like with the code, with some requirements to keep attribution (author comments etc) in place and notating in existing files that you've made changes. If you're trying to re-release it yourself, anyway, if you're just making changes to submit back to the community that's even easier
Well, that is true for the code, Cervator. Not necessarily true for the assets. If you want to release your own distribution of Terasology you technically need to either remove all the assets or seek permission from each of the asset contributors to use the assets that are part of your distribution. We still need to go through the process of either getting each of the assets in the engine/core licensed under something like CC or moved into modules.
I would urge you to consider using the modding support and creating your own module - wholly owned by you and which you can distribute as you like - or contributing back to the main project. Both of these have the advantage that you won't have to constantly merge your fork of the project with changes to the main development branch of Terasology.