A helpful way to think about it is to consider that game developers don't change the engine directly that they build their game on top of, instead they put things on top of it to add to it.
So, when building a new Star Citizen server before the change, they would take a server OS, put CE3.x on top of it, and then apply their changes over the top of that CE3.x install.
Now, they just take a Lumberyard CE3.8 install and apply their changes over the top of that install.
(although maybe they are using a SOE with everything baked in, I don't know who does their infra management etc, but the principle is the same)
Easy peasy