It was a while ago when I did it, but yes, swap to the CM3 first as the builds are different.
This is the scary part as you need to be able to see the eMMC partions on you PC (some people have trouble) so you can format it correctly.
Follow the instructions until you have a bootable CM3, then you can set LibreElec to update from the latest channel, which will download 8.90.008.