Heh, got my post out JUST IN TIME to be irrelevant. Here's the official list of legacy armies that will be Prime legal to start with: https://home.privateerpress.com/2022/08/18/legacy-armies-in-prime-arena/
I was right and wrong. Some of these are a bit baffling to me (Winter Guard and Storm Knights? We have new armies for those), some of these are obvious (how did I forget about Steelheads?), but hey, at least now we know!