Welcome to the TH community, AbSoLuT AL!
Even though the WD My Book Live is not compatible with Plex, I'd not recommend you to dismantle it and use it on the WD My Cloud EX. I doubt that the WD My Book Live has a WD Red, because it's an older external HDD model.
Since you wish to have one of WD's NAS systems, it's highly recommended to use a WD Red drive. This is our NAS/RAID drive and it's specifically designed to withstand the workload and the 24/7 environment of a NAS, thanks to its NASware 3.0 firmware. Here you can read more about that:
http://products.wdc.com/support/kb.ashx?id=ukRdFY
WD My Cloud EX devices are a part of our expert series NAS and have either 2-HDD or 4-HDD bays.
Since you want to use the NAS as a Plex Media server, I'd suggest you to look at our consumer personal cloud series: the WD My Cloud and he WD My Cloud Mirror (Gen 2). Both of them have the Plex app available and can be installed through the Dashboard. Here's a link to the products:
http://products.wdc.com/support/kb.ashx?id=1sd25f
You can choose from various capacities and I believe these might be a better home Plex server options for you. Once you have the new NAS, you can simply copy all your needed files from the WD My Book Live onto the WD My Cloud and still use both as you wish.
Keep me posted if you have any questions regarding them.
Hope I was helpful!
SuperSoph_WD