Perhaps Youtube is not your best of news/source material, it's better to get it from us instead.
And yes, OpenELEC is a dead project for at least 4 years now.
Yes, it will... -IF- you are not using a remote MySQL database. I'm also hoping that are you do not have your media (videos, music) collection on the same OpenELEC pc, but elsewhere on an external disk (USB, NAS, whatever). Right?
So when you are using the default local database, it's recommended that you do a Video Library Export, so all metadata will be stored in nfo files next to the respective videos (the same applies to the music collection). But first, you will need to enable the watched status and resume points in your advancedsettings.xml file, in order to save that info before you do that export. After that, do a Backup via OpenELEC's Settings Add-on. If you know how to also copy files from your OpenELEC setup manually, that would also be a bonus.
Since the system partition of LibreELEC has grown over time, you cannot simply upgrade from OpenELEC to LibreELEC. A full/clean install in mandatory. That said, since you are running such an old setup, it isn't a bad idea to start fresh altogether, as it will get you a cleaned up video library and refreshed thumbnail cache folder. As long as you did the Video Library Export, rescraping your media collection should be pretty fast as all metadata is already available. No need for Internet access there. And you will get rid of any unnecessary old clutter.
Oh man, wish I knew I could save watched status before I did the soft reset! All that info is gone so I know I have to do a new scrape and everything. Anyways, I opened up my htpc and found the good news is that my media is on a different hard drive. I unplugged the power to it and turned the system back on and it says the file is missing when trying to play any movie so I think Im good! So now, to just figure out how to install this thing fresh and I'll start over.
However, regarding covers, it looks like themoviedb went through an update and TONS of covers are missing. What gives?