adding new domain to existing website
I have an existing linode VPS server (Ubuntu 16 / Apache) and an existing domain (originaldomain.com). I want to change the primary domain to the new domain (newdomain.com), and no longer use originaldomain.com.
I went to the newdomain.com registrar, and switched the dns servers to linode (NS1,NS2,NS3,NS4.LINODE.COM).
I also went into the linode control panel, to DOMAINS, CREATE > NEW DOMAIN, and entered newdomain.com as the domain and chose to insert the default records.
Is there anything else I need to do? For example with the etc/sites-enabled / sites-available? Or custom records? Nothing has worked yet, though maybe it's still propagating?
If I'm understanding everything correctly you're looking to change the domain name of your existing website.
Assuming that is correct it seems like you're on the right track. DNS changes can take up to 48 to propagate so It's possible the records are still processing. You can use the link below to check the propagation status.
Depending how your web server was configured you might also need to update your Apache configuration file. To help you out with that, I've included some links below.