Fwiw the PoS network is already running, with $6 billion staked and 100,000 validators. Migrating the rest of the network is relatively simple, they're just being really careful with it.
The quickest migration is basically to upgrade both clients to talk to each other, and for the old client, instead of choosing blocks with the highest hashpower, choose blocks whose hashes are chosen by the beacon chain. Details here: https://notes.ethereum.org/@vbuterin/B1mUf6DXO
The next step is a more integrated system, described here: https://ethresear.ch/t/executable-beacon-chain/8271