For extra apps, it depends on the developer if they support Nextcloud, ownCloud or both. If there is only official support for Nextcloud, you can try to submit pull requests to keep it compatible with ownCloud as well, if this is too complicated or they don't want it, you can fork it (if the licensing allows it). If you are lucky, there are only minor changes:
Perhaps you just have to increase the owncloud min version. But each time you use it in newer versions, someone needs to test it.